Need help prop tuning....

[BadCo]

12.75" 16p prop on my 3 hole Tohatsu 90 is only pushing my 16' glastron V166 Starlite 40mph @ about 4000rpm. It's a glass semi-V tri hull and gets up on plain within 5-10 secs. This is a new motor (to the boat), the old motor was a 78 Johnson 85hp v4. It had a 17p on it and pushed the boat about 52-55 @ WOT (didn't have a tach to know rpm). I'm looking to maximize rpm/speed to get it running optimal. Any help is greatly appreciated and I thank you in advance

 

[Mr Efficiency]

Obviously your engine is nowhere near high enough in revs. It may help to know what the gearcase ratio is, first. The V4 would have been more CID, but there is something wrong if you are much slower.

 

[Mr Efficiency]

What Tohatsu motor is this ? 2S, TDLI, 4S ? I see the 4S has a gear ratio of 2.33 to 1, if yours has that ratio, there is something amiss with your calculations, you would not get remotely near 40 mph at 4000 rpm with that prop.

 

[BadCo]

The boat empty weighs 800 lb. With the weight of the motor and everything outfitted. It weighs roughly 400 more pounds. The gear ratio is the engine model is a 1994 90A. Here is the boat. I typically carry to 6 gallon tanks of fuel I have one large deep cycle under the splash well with the gas tanks and a second smaller deep cycle battery upfront under the captain's dash on the floor. Up in the bow compartment I have your typical anchor anchor lines and mooring lines. Sometimes I carry a tackle box and a few fishing rods. I hope that's enough information if not just ask and I will try to provide more info

 

[Mr Efficiency]

The ratio appears to be 2:1, going by that chart. So the propeller turns once, for every two engine revolutions. So. at 4000 rpm, the prop turns 2000 times, and if there was zero prop "slip", the distance advanced with a 16" pitch propeller, would by 2666 feet per minute, very close to 30 mph. But, for there to be thrust, there has to be propeller "slip", and in practical use, even in a very light boat, the speed would be no better than 26-27 mph, and probably less. So, your figures have a significant error, either your engine is revving much higher than 4000 rpm,or not going close to 40 mph. I'd reckon it would be getting well beyond 4000 rpm.

 

[BadCo]

I'm not 100% sure that my tachometer is correct... I installed a tiny tach to use for verification purposes but have yet to be able to check it against the tachometer on the dash. I use GPS to find out the speed.

 

[Mr Efficiency]

Well, you are likely getting much higher revs than 4000. The old OMC V4 was a fairly strong engine, possibly more urge than a 3-cylinder despite the rated HP, but I suspect you would not have been getting 50 mph-plus out of it, either, you do the sums with 17p and the 2:1 ratio box of the V4, 40 mph would be closer to top speed. I think an accurate tacho is needed. The 16p prop seems about right for your boat.

 

[BadCo]

I will verify RPMs and report back with my findings

 

[Mr Efficiency]

Good luck with it, I don't know how close the 90 Tohatsu was to the 90 Yamaha 2S 3-cyl, but the latter seemed not overly quick to my observations.