Weight questions

[Codeman]

When does weight start being an issue with a Jet outboard? Right now my boat is an empty hull and it GPSs about 23mph on rough water packing my 300lb+ butt and my 160lb or so buddy. I need weight in the nose to keep it down. How much weight can I add before I will start to see a drop in MPH? That being said I prob have some more speed in it due to the gap on the impeller and the liner really needs to be changed. I want to get this thing decked out to fish out of but I don't want to turn it into a pig.

 

[turne032]

what size is your motor?

How big is your boat?

The smallest amount of weight makes a huge difference if your watching a gps or timing your boat to reach plane. The two most determining factors that I have found to effect performance by the seat of your pants (hence, im not an expert) was boat set up and impeller and liner.

The boat set up or balance is very important. If you add a deck made of wood and a trolling motor, i dont think it would effect you very much. You may have to redistribute your weight, having your buddy ride in the rear with instead of riding up front to get weigh in the bow. but if you add a wood deck and wood floor, i think it would really hurt your performance. I am a huge fan of aluminum mesh flooring. Its lighter than plywood, and allows all the sand to fall through to the bottom of the boat and out the rear drain holes. rather than having to vacuum the sand out.

The jet impeller and liner is the most important part of performance. A good sharp impeller will pull more water through the pump and give more speed. I have expiermented with different size impellers with my 60/40, and noticed that my motor will pull a bigger impeller, and increase hole shot but sacrafice a little top end speed. I know that a 40/28 jet does not produce enough power to pull a bigger impeller.

Just my .02 worth

 

[Codeman]

Ok the boat is an old (read lightweight) 1848 and has a 1976 50hp Mercury with a tiller handle. At this point it is an empty hull. I have cleaned up and sharpened the impeller but the liner is grooved pretty bad and I know it needs to be replaced.

 

[Toby from MO]

Ok the boat is an old (read lightweight) 1848 and has a 1976 50hp Mercury with a tiller handle. At this point it is an empty hull. I have cleaned up and sharpened the impeller but the liner is grooved pretty bad and I know it needs to be replaced.

I’m no expert here either and can only offer personal experiences though for the most part I would agree with turne032. Though I think the definition of “huge difference” is relative to the user I absolutely agree with the balance. I also agree with the use of aluminum where possible.

My rig is an 1852 with a 60/40. I run a GPS on board at all times. On a lake, with no current and a typical load for fishing, I see speeds between 29 – 31 mph at WOT. River current makes a bit of difference as I have hit as much as 35mph heading down stream and as little as 26mph heading up…depending on the conditions.

When I add or remove small amounts of weight I see differences but typically nothing to be concerned over. For example, I remove approximately 100lbs (maybe more) worth of batteries and anchor from under the front deck of my boat. I gain at most .5 mph. A full load of fuel weighs about 100lbs…I know for a fact I see less than .5mph difference between full/empty.

During my recent trip to Taneycome to fish a trout tournament we GPS’ed the boat with a full load at 27.5 mph on still water. This included myself (230lbs), my partner (260lbs), 2 batteries, 12 gal fuel, 2 (large) fully loaded tackle bags, at least 15 rods, 2 20lb anchors, one 56qt cooler loaded w/food & drink, 2 livewells full of water, 1 20lb propane heater and enough cloths to keep an eskimo’s *** warm. Though it took a few more feet to plain I didn’t think a top end loss of 2-3mph was bad at all…considering I was carrying at least 450+ lbs additional weight vs. “normal”.

If I were you I would get your liner replaced ASAP. Your boat being a 48” bottom and having nothing in it leaves me scratching my head as to why it only runs 23mph. I don’t know much else about your rig but IMO my boat has to be heavier than yours, not to mention it is 4” wider, and my old 40/28 Johnson would push mine at 23-24 WOT, this was with 3+ years on the impeller and liner.

Are you sure your engine is getting up into the 5200-5500 rpm range? Have you checked compression? You might also adjust the engine up/down on the transom.

Toby

 

[Codeman]

Thanks for the help guys. I wasn't too disappointed with the MPH. I am taking into consideration that the motor is 33 years old and I am sure it hasn't lived an easy life. That being said I was hoping for closer to 30. The liner is junk in my opinion I talked to my neighbor the outboard mechanic and he said it would be fine till I had a little more of it sorted out. At the time I wasn't even sure if this thing would run. I have not put a tach on it as of yet but I have one laying in the shop that should work and I plan to try it in the next week or 2. As far as the balance with me moving form the back to the front picked it up around .5 MPH with my buddy driving. So I realize that it will make a difference.

I still have a lot to learn and a long way to go on getting this thing where I want it. Main reason I asked this I want to get my decks added and get it close to where I want it.

Thanks for the help.

 

[turne032]

liner is fairly inexpensive 50$
impeller is a little more 185$

Those older mercs are good motors, but they are crossflow motors that did not perform as well as late model motors. Merc loopers performed much better, but they did not produce them untill the later years. Johnson/Evinrude motors (my opinion (which isnt much)) Performed much beter with jet pumps. Merc did build a 60/40 loop charged motor that performed very well out of the box. Really performed better than Evinrude/Johnson out of the box.(only difference was Evinrude/Johnson has a detachable head that could be taken off and increased compression where Mercs had a head that was cast in place). Therefore Evinrude/Johnson could be built to run a little better/and cheaper. Those older Merc motors can be built to run better, you would have to talk with a an expert.

The biggest difference was the pump. A 60/40 motor (2 stoke) had a bigger jet pump than a 40/28. Therefore could handle a bigger boat better. Top end speed wasnt as much difference as was the amount of weight you could haul. The bigger pump your motor has the more water volume would come through, and the heavier boat you can have.

I agree with Toby from Mo, I would think your motor should put out more. My brother had a 40 merc on a 17x 48 boat that would do 26 mph with gps. I would definately get a new liner, and if i could... get a new impeller. If you have it sharp it helps, but time wears the blade edge back and creates a hook in it which hurts performance. Get the impeller as tight as you can against your liner.

once again i do not claim to be an expert, just exercising my $.02 worth.

I do love this forum to talk about jet outboards, i think everyone is great.

Turner

 

[Codeman]

Thanks guys. The new liner will be soon (checked on it today) but I might have to wait on the impeller. I need to get some other things done first. This thing was a basket case when I got it and I'm proud to have it up and running to this point. As far as tweaking I'd like too but with this old motor its not my real intentions. Believe me I'm all about going fast on pavement but this is my first round with boats. My neighbor (the outboard mechanic) told me these old Mercs are tough as nails but not to expect to keep up with the newer motors.

 

[Codeman]

Update: I not sure what changed that much but with only me in the boat today on very rough water I was able to hit 27 MPH on the GPS. That was with a full tank on fuel (6 gals) one battery in the back with me and one up front as well as a 65-70lb sand bag for ballast in the nose. It seemed to take for ever for the nose to settle down and the boat to run flat in the water. I felt like the rear was coming up on plane Ok but the nose was up there.

 

[lowe1462]

Can you trim the motor down anymore? Maybe you could do that to bring the nose down, and not need so much ballast (You will probably always need some).

 

[Toby from MO]

If your considering building a deck in the front, wood construction may help you. I'd also consider trolling batteries, anchors, etc... stored under the front deck would be a big help.

Though I still go back to a the liner/impellar. A new liner with new or sharpened impellar will do wonders for your hole shot.