Semi Displacement Power

[fritzdfk]

I am looking at a 26' semi-displacement boat with twin 50hp Mercury 4-stroke outboards. The outboards are not the Bigfoot model. I am not interested in more than displacement speeds. The 50 hp Mercurys running at 2240 rpm will push the boat at 6 knots burning 1.4 gallons/hour. This is decent economy. I wonder if better economy could be had with twin Yamaha 25hp high thrust 4-strokes at 6 knots. The smaller high thrust Yamahas turn the same diameter props as the bigger Mercurys. One disadvantage however is that the smaller Yamahas are not EFI.

Originally Posted by fritzdfk

I am looking at a 26' semi-displacement boat with twin 50hp Mercury 4-stroke outboards. The outboards are not the Bigfoot model. I am not interested in more than displacement speeds. The 50 hp Mercurys running at 2240 rpm will push the boat at 6 knots burning 1.4 gallons/hour. This is decent economy. I wonder if better economy could be had with twin Yamaha 25hp high thrust 4-strokes at 6 knots. The smaller high thrust Yamahas turn the same diameter props as the bigger Mercurys. One disadvantage however is that the smaller Yamahas are not EFI.

There sounds something wrong to me.

My 28ft semi does 14 kn with a 4cyl. 75hp Yanmar diesel, and 6 kn with about 25% - 30 % power. So, I would assume (not knowing your boat), you should at least do the 6 kn with just one of those Merc.s and still have some reserve.

Anyway, you should be better away just selling one of your outboards and call it a day. (maybe a better matching/adjusted prop will help further?)

Regards
Richard

[PAR]

I'd have to agree, that boat of yours must be pretty heavy to require 100 HP to do 6 knots. I have a 27' boat, weighing 4,500 pounds, with 185 HP that does 37 MPH.

What make, model and year boat is it? In fact, some quick calculations and assuming a 5,000 pound boat with marginal hull form and moderately high drag and prop slip, you should still be running in the high teens, if not the low 20 MPH range.

To answer your question, as a rule a single will have better economy then a twins.

[Joakim]

Originally Posted by PAR

I'd have to agree, that boat of yours must be pretty heavy to require 100 HP to do 6 knots.

It doesn't! The engines are running only 2240 rpm and take 1.4 gallons/hour. 1.4 gallons/hour equals to something like 15 hp.

Originally Posted by Joakim

It doesn't! The engines are running only 2240 rpm and take 1.4 gallons/hour. 1.4 gallons/hour equals to something like 15 hp.

True...........