Help with atypical motor setup

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PunkR0ckz

Active member
Joined
Aug 3, 2023
Messages
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LOCATION
Montreal, QC
Hello everyone,

First and foremost, I am pretty new to here, as I just acquired a few days ago a Starcraft Offshore 18.5' V-hull from the 70s.
I am pretty stoked about the decision to buy this hull, it seems like a great platform for my needs.
This boat came with what seems like the original motor for it, a 1970s Evinrude 115hp v4 2-stroke
One of the issues I have is that most places where I go, they require a speed limit of 30mph and a maximum of 25HP, with the motor being a 4-stroke or electrical, otherwise they will not give you a season pass.
Another issue is that in 85% (or more) of the situations, I will solely be using my 2006 Mercury 25hp 4-stroke, the other times (15% or less) would be the 115hp. It would mostly be for when there is big winds, heavy rain, hailstorms, thunderstorms or when I need to do 10+ miles on the water.
Therefore, I am considering my "main" motor as being the 4-stroke and the 2-stroke as being a "emergency"/"kicker" motor.
I will be reworking my transom and splash well in accordance to whatever setup I end up going with, and I will get rid of the plywood core and make it an aluminum core and strengthen it accordingly as well.
In the future, I will be adding a door to the transom in order for me to easily get in and out of the boat, alongside a swimming platform, a ladder and under that a transducer to keep it protected from us going in and out of the boat.

I searched a bunch online and I couldn't find much of an answer to my questions, so I hope someone can help me out on here.

Granted that the 115hp is about 400 lbs, the 25hp is about 125 lbs and I am about 350 lbs,

1- Would installing the 115hp to the port side and the 25hp to the starboard side, off center by at most 15" (less than 30" from motor to motor) cause any issues in regards to steering/tracking?
Both motors would be linked together with the steering wheel, so they would always turn together. It is cable-driven, not hydraulic or electronic, and I don't intend on changing this.

2- Would installing the transducer and swimming platform/ladder to the port side cause any issues with the motors? (I'm thinking here mostly about prop wash or water splashes from the prop over the swimming platform)
The main reason why I'd like to install it there is because I am stronger with my right hand to pull myself out of the water, and the "rail" would protect people from falling towards the outboard in case there is a wave coming by.

Otherwise, what would you guys suggest, keeping in mind that my "main" would be the 25hp and the "kicker" would be the bigger 115hp? My thinking was to offset the biggest motor to the port side as little as possible so that I have enough space to the starboard side and port side to have storage and a door to the swimming platform and so that it balances out the hull better (the 115hp is about the combined weight of the 25hp and myself), but if I need to offset them more towards the outside of the boat and have a door in the middle, I'll do it. If neither of those options is a good one, I'll stick with the regular 115hp in the center and 25hp on the port side, even if my 25hp would be the "main" outboard.

As you'll see in the pictures, it has a pretty big splash well, so I want to square it off and put storage below the side that wouldn't have a door, mostly life jackets and a cooler on wheels, which takes up quite a bit of floor space for no good reasons and I do not intend on buying those brackets that puts the motors further away from the transom, the boat barely fits in my garage as is.

Thanks y'all for the help.

Note; As I do not have a picture of my actual transom, I took one off of the internet, that is why the motors does not match. Mine is the one with the cowling, the other one is off of "sstout"'s post on another forum.

Edit; By "swimming platform", I am solely speaking about those tiny 2ft x 2ft-ish platforms with a ladder and a rail attached to it, and if I have to scrap the idea of having a door in the transom, I will, but I'm thinking it can be done. I haven't measured it though.
 

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Two linked motors, a swim platform, a ladder and a door seems excessive to me on that transom
As I said, I will be redoing the entire transom, so unless you're talking about width, it is possible, I haven't measured yet (it is in storage because I'm in Canada and winter's here...) but I'm sure I can make it work. I just don't know what would be the best setup for my use case scenario.
 
What you are asking is very unique in my experience. Thanks for bringing here and welcome to these forums.

I agree with @Billy H's comments above. I think one of the reasons you haven't found anyone doing this is that is too much to expect of that boat. I assume you are thinking of cutting away part of the splash well and cutting into the transom? I just don't think it is practical. You would be giving up too much structure needed for that big motor.

I also can't see having the 115 that far off center. I'm not sure what it would do at speed, but suspect it wound't be good. Maybe others who know more than me can address this.

I would also think about what will happen to the boat's trim by having the combined weight of 115 & 25 HP motors hanging off the back. If the boat is capable of that, then life is good. The reason I mention it is most boats I see that size are using a 9.9 or less HP kickers.

One other thought -- if most of your lakes require a four-stroke with 25 HP max, are they going to allow you in with the 115 two stroke hanging off the transom?

My thought is scrap the transom door idea. Keep the 115 centered with the smaller kicker to the side with linkage to the big motor for steering. Most always the kicker is on the port side. I don't know the reason why that is. Might be worth researching. If you want a swim platform put it on the other side. You might be able to come up with a way to put a level deck on top of the splash well to make it easy to scoot across the transom to the platform.

Sorry if this is a big buzz kill. I would be happy to be proven wrong.
 
Regardless of how your using it the 25hp is still going to be mounted as the kicker as such your biggest concern is the weight. The biggest motor I've ever seen hung in that fashion was a 15hp on a big fiberglass ranger. Its not just about the transom being strong enough but the effect of all that weight that far back. When running the 25hp it won't track like it would if it were centered. The transom door idea is often seen on large offshore boats with inboard engines I'm certainly not an engineer but in my mind on a boat that size with a outboard its asking for problems. It doesn't sound like you run all that far have you tried the boat with a 25hp to see what it does? It seems like a lot of work to occasionally use the bigger outboard to run 10miles. As for the lake letting you in with the big outboard attached it all depends on who runs the lake? Which lake or lakes do you plan to fish with those restrictions?
 
To answer both of you ( @LDUBS and @flatheadsteve );

I haven't completely inspected the structure of the boat as it is either pouring freezing rain or snow around here on the daily (Thanks Canada).

My original idea was to completely remove the dual layer of plywood in the transom, replace it with a structure made out of 1.5 x 1.5 aluminum tubes, which would also have the cutout for the small door (it would probably be a door that's like 20 inches by 20 inches or so (I'd have to measure it) and then, I would make another structure where the "front" of the splash well is. That structure would have storage for my cooler on wheels, an anchor, 2 or 3 life jackets for my friends/family, and a gas tank for the auxilary outboard (as I already have a huge subfloor aluminum gas tank). It would also be connected from gunwhale to gunwhale, and to the floor. Where the door is, there would be a small step so it is achievable. So, structurally speaking, I am not afraid at all of putting both motors there, I know it will be plenty strong enough to support both motors, but I just don't know how well it would work out (especially tracking straight and listing).

The big subfloor aluminum gas tank will be in the front of the boat near the cockpit to offset the weight added to the back by my structure and the 2nd motor.

Also, no, I haven't tested the boat or anything; I bought it less than a week ago, the 115 isn't operational, neither is the boat (holes below the water line), and I have never done long hauls because we've (me and my uncle) never had an outboard strong enough to carry us through the wind/waves or bring us somewhere far quick enough to not waste our day. We usually ran a 9.9 or a 15hp on the back of a 16ft fiberglass boat reaching its weight capacity... so we were barely going on plane everytime. If I could have the 115, it would make this possible and bring more possibilities for fishing, which would be very pleasing for me and less boring than the same old spots I've been fishing for 20 years...

Also, to answer the lake thing, they are mostly cities that actually drink water from the lakes, or private lakes which a bunch of beaches and whatnot, so they don't want fast boats there and everything. You have to show a proof of residency at the gate to be able to get there, you have to show the sticker given by the city after you filed in a report and sent pictures and they attest your boat is ok to go there, etc. The only circumstance in which they will allow you to use a faster engine that isn't a 4-stroke (in my case the 115 v4 2-stroke) would be when there is a risk of injury or death (someone cutting themselves open with a knife while filleting a fish, hook to the throat, severe winds/hailstorm/rain/thunderstorm that is incoming and will hinder visibility) in those cases, they will send you an SMS and you have to get back (your cellphone number has to be listed in the paper works and you have to show it at the gate with a full charge or they won't let you in at all, yes, its weird, but that's how they work cause they had a few incidents happen in the past and they take it much more seriously now)

I think I answered everything, sorry if I forgot something.
 
You mentioned your weight to be around 350#, if your back there with all the other stuff, your boat won't handle it well. If the big motor is offset, your boat will not track well. If there was ever a boating accident involving your modified boat they way you describe it, I believe the insurance company would find your boat well outside a safely designed watercraft. I would rebuild the boat and motor and pick up a smaller tinnie with a 25 hp or smaller motor. Afterall, a man can't have to many boats !!
 
You mentioned your weight to be around 350#, if your back there with all the other stuff, your boat won't handle it well. If the big motor is offset, your boat will not track well. If there was ever a boating accident involving your modified boat they way you describe it, I believe the insurance company would find your boat well outside a safely designed watercraft. I would rebuild the boat and motor and pick up a smaller tinnie with a 25 hp or smaller motor. Afterall, a man can't have to many boats !!
This isn't a tiller, it is a dual console, so I wouldn't be "back there" with the motors, neither would be the main gas tank and batteries, so the weight distribution would be decent. I found online some boats that had outboards off center, they are deep V hulls just like mine, and they said it tracked great (I read somewhere that offset towards starboard is better because of prop trying to lift off the port side). These boats are "Tolman Alaskan Skiffs" and a book was written mentioning how much offset you can put based on the 3 types of hull they give you plans for and your HP rating.

What do you think I would modify that would make it less reliable than it already is? I'm reinforcing 99% of what is already there by adding structures below it and strengthening the transom. The door wouldn't have any effects on the transom at all as it would be just above the horizontal line that the outboard sits on, and there would be framing on each side, so the walls of the hull wouldn't be able to "cave in" (for a lack of a better term).

By the way, if I'm trying to jungle with that "mess", it is because I can't have another boat. I don't have the money for it, and I don't have the real estate for another one. This one is perfect for everything, I just need to figure out the motors or wait until I find one hell of a deal on a 40-50hp 4-stroke (they're upwards of 8000$CAD around here, used... a 9.9 is about 1500$CAD to 2000$CAD lol...)
 

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I understand you are not near your boat at present...but a picture of your actual boat would help us help you tremendously.

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The one with the mercury at the back (no cowling) isn't mine, but it is the same exact transom and boat model (I don't have one from this angle). The other 2 pictures are of mine (motor with cowling and from inside the boat)
 

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As you can see by the pictures, I would like to reduce the depth of the splash well by about a feet to bring it closer to the cowling, as this is wasted space (cowling can be easily taken off even when fully tilted, and if I add a swimming platform, it'll be even easier), which in turn will give me more console-to-stern space, which would be greatly appreciated, and if I redo the entire splash well shape, I'll make it more square, instead of tapered sides, so it is easier to make a full height storage for my cooler on wheels, life jackets and anchor on one side, which would also have the "kicker", and on the other side, I'd do the door.
 
Well, you seem to have answered you own questions. To me that still seems like a lot happening for that transom.

Here is another thought. Why not keep the 115 centered and mount your 25 kicker on the transom right next to it. Looks like there might be room. As said earlier link the 25 to the bigger motor. When you turn the wheel the big motor and small motor turn together, hence steering when using the kicker. When using just the big motor tilt the kicker up. This is a common set up.
 
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Well, you seem to have answered you own questions. To me that still seems like a lot happening for that transom.

Here is another thought. Why not keep the 115 centered and mount your 25 kicker on the transom right next to it. Looks like there might be room. As said earlier link the 25 to the bigger motor. When you turn the wheel the big motor and small motor turn together, hence steering when using the kicker. When using just the big motor tilt the kicker up. This is a common set up.
I will obviously be linking the 2 engines together, as far as steering goes as they are both console-driven engines.

However, I'm still not decisive as to what I would do with the engines themselves. Knowing that Tolman's Skiffs does it doesn't explain why, how much offset I could get by with, or the pros/cons about it.

A lot of people seems to agree it is best to mount the engine centered or up to 2 inches towards starboard (depending on HP), but this creates listing issues, and in my case, they are very noticeable as I am a big boy.

Some people seems to say it is best to offset the main towards the port side to balance out the hull and negate the need for trim tabs, but it would be harder to steer.

Some people seems to say it is best to offset the main towards the starboard side to make steering easier and slightly negate the list caused by the outboard being centered. (Prop's water displacement wants to raise the port side)

And a very few people contradicts either of the offset positions "No, it isn't harder to steer, no it doesn't balance out the hull, you'll be doing donuts if you offset the main, etc."

So far, I am leaning towards a 1" to 1.5" offset of the main towards the starboard side, mounting the "kicker" as close as possible to the main and cramming a homemade swimming platform in the space left on the starboard side with a ladder and a transducer. I might not do the door until I am 100% good with the setup.

I'll give it a few days/months in hope that someone has other helpful hints, comments or even real life experiences to share with a similar situation. I can't work on it this winter at all anyway.

Feel free to post any other questions or comments you might have, it might get my hamster wheel spinning and help me (or you) come up with a solution hehe.

Thanks y'all for the replies this far!
 
Congrats and welcome to the forum!

Thus far, there has been great advice provided on your project.

Do not offset motors for reasons mentioned.

Have you researched (or a plaque on boat) and found the specs for MAX outboard HP and weight? This is very important to know, especially with 525# of motors at transom.

If the boat meets the weight specs of 115hp and 525#, I would leave the 115 centered and 25 off to port. Swim step/boarding ladder and transducer to starboard.

The cutting of transom for a door.....may weaken transom, unless engineered, spec'd, and custom built for one. Transom may not be suitable for this type of modification. I believe it would require a highly skilled TIG welder and highly skilled fabricator.

The BEST solution you mentioned.....a mid size (50hp-4 stroke).

In the meantime, see what you can find out about the MAX HP and weight specs for your precise year/model.

When you get the opportunity, pics of your boat would be great.
 
Are you serious? Why try to reinvent the wheel when their are so many boats out there. Surely one suits your needs. This idea should be quickly tossed to the curb IMO.
 
Congrats and welcome to the forum!

Thus far, there has been great advice provided on your project.

Do not offset motors for reasons mentioned.

Have you researched (or a plaque on boat) and found the specs for MAX outboard HP and weight? This is very important to know, especially with 525# of motors at transom.

If the boat meets the weight specs of 115hp and 525#, I would leave the 115 centered and 25 off to port. Swim step/boarding ladder and transducer to starboard.

The cutting of transom for a door.....may weaken transom, unless engineered, spec'd, and custom built for one. Transom may not be suitable for this type of modification. I believe it would require a highly skilled TIG welder and highly skilled fabricator.

The BEST solution you mentioned.....a mid size (50hp-4 stroke).

In the meantime, see what you can find out about the MAX HP and weight specs for your precise year/model.

When you get the opportunity, pics of your boat would be great.
The boat is spec'd for 8 person, 1900# max (person, gear, motor) and 125hp max, and I don't know its year, there is no HIN and the serial number engraved on the plaque doesn't tell the year, as far as I know... I found a few that says its the first 2 digits, but that be in the mid 90s, which doesn't make sense as they stopped making the Offshore model in the early 70s from what I know.

From what I found online, this Starcraft Offshore-V in a outboard configuration weighs 800#, in the I/O configuration it was around 950#. It'll probably be around 700-725# when I am done with it (removing the seats at the front, most likely removing the passenger console, removing the "transom sliding doors" (I don't know if they had a proper name), etc.
 
Are you serious? Why try to reinvent the wheel when their are so many boats out there. Surely one suits your needs. This idea should be quickly tossed to the curb IMO.
Why wouldn't I be serious? I live in Canada, near Montreal. We don't have those type of boats with transom doors, outboard brackets, etc., around here.
This is also the longest and widest boat I could find that supports the most amount of weight and number of people.
The "more suited" boats with transom doors aren't really suited for fishing, and they are over 40 000$, I paid less than 500$ for this boat.
And, to my knowledge, there is nothing wrong with asking questions, unless you're stuck in a "thinking inside the box" situation, I'm the type to think outside the box and try to come up with weird things that makes everything work.
Heck, Stabicraft does it, and their smaller boats are narrower (by a few inches) than mine, so I'm sure there is a way to do it, and as far as the engine offset question goes, there is a good reason to it, it could help me get rid of the natural listing of having too much weight towards the starboard side, thus requiring trim tabs, which is an expensive product around here.
 

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