Engine Performance

[gerrytok]

I recently asked the question about engine performance and had several replies. I only just got round to fitting the tacho/rev counter, I recorded 5000 revs and topped out at 24 mph/20knots. The engine is Mariner 40hp 4stroke EFI, mounted on a Shetland Suntrip 535 on fresh water. I am hoping that there is adjustment in the throttle cable to achieve WOT .
The propeller is a Black Prince( Diameter 10 1/4 ) ( Pitch 14) ( 3 Blade RH Aluminium.)
I am getting the boat to plan okay but What speed should I achieve with wot with 2 passengers

[davidemania]

On my 498 with a Tohatsu Mega (should be around 50/55 HP) I get to 24-25 Knots with 2 adults onboard, your boat is bigger than mine so your maximum speed doesn't seem too unreasonable. What are the maximum revs for your motor?

BTW this question is in the wrong forum, may be the admin can move it to the proper place.

ciao
Davide

[gerrytok]

Thanks for your help, I realise I should have put the question on a different page. I am getting 5000 revs WOT

[davidemania]

Sorry, at first I read the data for your propeller in the wrong way... a 14 inches prop look way too long for a 40 HP on your boat, may be you should think about mounting a shorter one. But for this I'd wait for an advice from the forum experts, I suppose it could also depend on the gear ratio you have inside your motor.

[kinnon88]

I have a mariner 40 EFI on my hobby. Pretty sure I'm not getting 20 knots. Maybe 12-15.

[davidemania]

This is strange, the hobby should be very alike the 498, on which I get 24 Knots with 4 people and 50 HP. 15 knots is barely enough to plane, with only the pilot onboard.

[kinnon88]

Yeh, I'd say she was just about planing. The engine is only about 2 and a half years old. Anything you would suggest I check?

[davidemania]

I do not know for sure, you could check motor height (anti-cavitation plate should be slightly lower than the lower point of the hull), trim settings/weight distribution on your boat, maximum revs you get at WOT compared to the specs. If all these things happen to be right I'd have the motor checked by an expert to see if it has any problem affecting its output power.

Unless (but you wouldn't want this to happen) the foam fillings of your boat are soaked with water adding lots of extra weight. We could try measuring height over water on your boat and mine to have an idea of what could be their weight.

[kinnon88]

Thanks for the quick response! I will have a look at the engine when I'm near the boat. As she will likely not be in the water until next year, I'll have to wait to check the displacement. Do you know how get to the foam to check for water?

Cheers again

[kinnon88]

Not the best example but this is us going flat out. At the start of the video in the link above shows much the same pace.