Calculated Prop Slip on Jon Boats - What is a reasonable number?
I picked up a new project boat a little while ago. It is a DMI 1648 mod-vee jon boat (welded seams, but rivited supports), with a '98 25HP Merc 2 stroke.
With the merc prop (13P), I am getting 5600 rpms (wot rpm range 5-6K) with a gps speed of 26mph. This is two adults plus some fishing gear. Manual trim settings were tested, and best seems to be parrellel with bottom, but about 1.5" below. Raised it 0.75" with no measurable improvement. Water still going over top of cav plate. I cant raise it any more without drilling holes in transom or using a jack plate.
Using a calculator, and specified gear ratio, I get a 16% slip. That seems a little high, but this is my first jon boat. Speed seemed reasonable to me, based on semi-v and v experience. I have seen some people reporting speeds of 30-31mph with similiar boats and motor combos. Many people don't seem to use tachs on small motors, that and not having the engine gear ratio, kept me from being able to make a comparison. BoatTest.com doesn't seem to test these small combos, either.
My next testing will be with added weight to make sure I will still have decent performance after adding trolling motor, battery, front and rear decks, etc.
I picked up a new project boat a little while ago. It is a DMI 1648 mod-vee jon boat (welded seams, but rivited supports), with a '98 25HP Merc 2 stroke.
With the merc prop (13P), I am getting 5600 rpms (wot rpm range 5-6K) with a gps speed of 26mph. This is two adults plus some fishing gear. Manual trim settings were tested, and best seems to be parrellel with bottom, but about 1.5" below. Raised it 0.75" with no measurable improvement. Water still going over top of cav plate. I cant raise it any more without drilling holes in transom or using a jack plate.
Using a calculator, and specified gear ratio, I get a 16% slip. That seems a little high, but this is my first jon boat. Speed seemed reasonable to me, based on semi-v and v experience. I have seen some people reporting speeds of 30-31mph with similiar boats and motor combos. Many people don't seem to use tachs on small motors, that and not having the engine gear ratio, kept me from being able to make a comparison. BoatTest.com doesn't seem to test these small combos, either.
My next testing will be with added weight to make sure I will still have decent performance after adding trolling motor, battery, front and rear decks, etc.