Boat is slow

[Ndheep]

Just wondering how fast I should expect my holiday 16ft boat to go powered by a mercury 40 hp 2 stroke. Motor runs good not sure on rpm at wot. Have a tach on order. With myself in the boat it will plane out but with 3 people in it wont get all the way on plane. It doesn’t have power trim. Have changed the trim pin and the best it could get to was 17mph at wot. Just wanting to know if that is what I should expect it to do or if I should be looking into what is keeping it from going faster.
one thing I’m going to try is taking the troll plate off and see if that is hurting it.
thanks for any help.

 

[racerone]

Well a 40 HP ( in good condition ) is bare minimum.----I think you need 70 HP for your boat.

 

[Ndheep]

Yeah I get that. Looking for a bigger motor. Picked 2 50hp mercurys yesterday. One with power tilt and controls. Hoping that will work better.

 

[BWR1953]

The 1975 and 1978 Holidays are rated to 85HP, so if yours is similar, then you're definitely way underpowered with a 40.

http://boatspecs.iboats.com/Starcraft_Marine__Holiday_16__1975/bp/66b86452

Minimum for "fair" performance would be a 60HP or better since 75% of max is considered minimum for adequacy.

 

[Ndheep]

The coast guard plate says 80hp max for boat. I bought it from the original owners son. So it came with the 40hp mercury. Well I guess I should look for even bigger than the 50 I picked up yesterday

 

[Ndheep]

Would the trolling plate mess with the performance of the prop? I was thinking that might get air pockets or something like that.

 

[ifishmuskie]

What size prop is on it?

 

[Ndheep]

I can't read the numbers very well. So not sure. This is all that is on it

 

[Watermann]

I can't read the numbers very well. So not sure. This is all that is on it

Looks like there's a problem with that prop or someone didn't torque it down, the face of the prop hub should not have all that circular scarring. Those numbers don't mean anything to me, it may have the pitch and diameter numbers stamped on the outside of the prop barrel.

A 50 horse would be an improvement over the 40 especially with the TnT and most likely need a 15 pitch prop to achieve it's max RPM at WOT.

 

[Ndheep]

Yeah I wondered about the face looking like it spun or slipped going to order a new one when I figure out a starting point for pitch and size of prop.

 

[Ndheep]

What would be a good size prop to start with. the one on it now ihas about a 10 inch diameter. know idea on the pitch. I just want the boat to be able to plane out with the family on board. Don’t need to set a water speed record.

 

[Ndheep]

Took the prop off of my 50hp it is a 10.38inch with a 13 pitch. Hoping that this will at least give me a starting point. Going to take it out tomorrow and see.

 

[jimmbo]

A 13in Pitch sounds a bit big for a 16 ft boat. I would predict a 10 or 11 inch being a better choice. Your engine has a WOT range of 5000-5500 rpm
The Scratches on the Face of the Hub, indicate a lot of Slippage between the Prop and the Thrust Washer it is up against. Possibly the Prop Nut isn't tightened enough, or the Rubber Hub is Slipping
The Number on that face, is likely from a Prop Rebuilder.

 

[SHSU]

What would be a good size prop to start with. the one on it now ihas about a 10 inch diameter. know idea on the pitch. I just want the boat to be able to plane out with the family on board. Don’t need to set a water speed record.

A 40HP to get a 16ft boat loaded down with gear and people on plane is asking a bit much in my opinion. You got to fight the weight of the boat to get it up on its own wave to plane. A whale tail might help, but again 40 HP is way under powered. Past comments I have seen from others is to max out the HP rating of your boat if you want performance.

SHSU

 

[Ndheep]

Might have been repaired at one time. On the look out for a bigger motor. Had a line on a 70 evenrude but they sold it.

 

[g0nef1sshn]

My 16ss with a tiller 2015 60 merc command thrust hits 32mph solo. 30 with me and my dad (were not skinny people lol). I had a older 40 merc 2stroke that pushed it pretty good. But blew that one up hitting a submerged stump on its first duck hunting trip.

 

[airshot]

I have a 16' Sylvan with a 40 Merc two stroke (1992) and it will do 31 mph with one, 28 with two and 24-25 with three adults. My boat is heavy, carries 4 big batteries, boat is just under 1000 lbs with no passengers. I run a 10.5 x 13 pitch prop, with just me it turns 5650 rpm, with two about 5500 rpm so prop matches the boat. Either your hull has issues or your motor needs a tuneup or your tach/ speed is off. I mostly fish alone or one passenger so 40 hp is plenty of power for me, but, everything needs to be set up correctly...and yes, the power trim adds about 3 mph when trimmed correctly.

 

[jimmbo]

I have a 16' Sylvan with a 40 Merc two stroke (1992) and it will do 31 mph with one, 28 with two and 24-25 with three adults. My boat is heavy, carries 4 big batteries, boat is just under 1000 lbs with no passengers. I run a 10.5 x 13 pitch prop, with just me it turns 5650 rpm, with two about 5500 rpm so prop matches the boat. Either your hull has issues or your motor needs a tuneup or your tach/ speed is off. I mostly fish alone or one passenger so 40 hp is plenty of power for me, but, everything needs to be set up correctly...and yes, the power trim adds about 3 mph when trimmed correctly.

2 completely different engines, so that is an Apples to Oranges Comparison.
Crank vs Prop Rated
2 cylinder vs 4 cylinder
33.3cu. in vs 44cu. in.

 

[Ndheep]

So I put that 10.38x 13p on it pulled great but I have no idea what rpm it was truly running at. I need to get a real tach this one that wraps on the plug doesn’t seem to be consistent. at wot at first was at 4250 and maybe that is right but a bit later it said 2400. So not sure. got a bit more speed. Topped out at 19mph. It planed out and ran good. Boat was a little lighter today so that might be why it planed out.

 

[jimmbo]

As I said before, your engine has a WOT range of 5000-5500 rpm. If all you are getting is 4250, and the engine is running properly, then you are risking serious engine Damage, as in Melted Pistons. A lower pitch will bring the Rpms up to where the motor is making its rated Power, and you will likely see a Speed increase.