which engine for semidisplacement hull

[malagaoth]

i run a "stewart-stevens" 20 ft fast fisher with a semi displacement hull and a 60hp Yam. at the moment at WOT i get 12-15mph on the GPS and with my 5hp emergency engine i can get 4-5mph. now the question...i am thinking of repowering with a 40 bigfoot by this i hope to save a bit of fuel without dropping too much performance what do you think?

 

[umblecumbuz]

Re: which engine for semidisplacement hull

Hi Gary,<br /><br />Theory says that you should get roughly 10-12 mph from an equivalent 40hp. motor. That's not even a ballpark figure though, because there are just too many variables. <br /><br />And it doesn't tell you anything about fuel consumption. But remembering that even two equal horsepower motors can give varying consumption, there's only one way to find out for sure!

 

[tommays]

Re: which engine for semidisplacement hull

i cant see saveing any money unless the old motor is a carb style VS a newer style EFI of some type<br /><br /><br />tommays

 

[andymach23]

Re: which engine for semidisplacement hull

Gary<br /><br />What is your WOT? Are you getting close to Max revs for your motor. If not, a new prop might improve things.<br /><br /> I would not like to drop down on hp. 60 to 20 is a large % drop. This may make your boat difficult to plane.<br /><br />I have a 17ft semi displacement Hardy with a 40 hp 4 stroke Yam. 40 is the max for the hull. It goes really well since I got the correct prop. It is also nice to be able to drop the revs back and keep a useful planing speed.<br /><br />Alot of people fit 50hp big foot motors to the Hardy 20 Pilot. Also a semi displacement heavily built boat. 50 is the bare minimum for this boat for the sea. Once you have 2 or more people on board it will not plane with this hp. A 75 or 80 gives best results with this boat once trim tabs are fitted also.<br /><br />A common consensus is that most boats are happiest at or near the max for the hull. People never really complain that they over powered their boats if they have the max hp. They usually complain if they think they under powered.<br /><br />Good luck<br /><br />Andy

 

[andymach23]

Re: which engine for semidisplacement hull

Sorry, should have said 60 to 40 is a large % drop.

 

[malagaoth]

Re: which engine for semidisplacement hull

i should make it clear that even with a 60hp the boat never even comes close to planning. speed is not an issue however as i rarly go more than 10 miles from home and view the journey as nearly as important as arriving. unfortunately i do not have a tach on the enging but from about 3/4 throttle up it appears that the difference is chiefly in noise output rather than thrust! (10mph vs 12mph) and a very noticable increase in fuel consumption at a guess id say mpg halves between 3/4 and full throttle. i just thought that since i run at 1/2 to 3/4 throttle i might as well get a smaller engine, the advice here seems to be either stay with what i have or even go up to perhaps a 75hp.

 

[Paul Moir]

Re: which engine for semidisplacement hull

Most outboards are most fuel efficient at somewhere around 2/3-3/4 throttle. I think if you went from a 60hp at 3/4 throttle to a 40hp at full throttle, you'd actually see a drop in fuel efficiency.<br /><br />For example, have a look at the GPH vs RPM consumption here:<br /> http://www.yamaha-motor.com/assets/..._2stroke_hpmidport_al_alw-1652vvsc-60tlrc.pdf

 

[mjbrueck]

Re: which engine for semidisplacement hull

For a semi-displacement hull, a slower turning, big prop (like on Merc's Bigfoot) would be more efficient than a smaller prop. If it's time to replace the engine, I think the bigfoot would be a good choice. If you want to keep the Yamaha, I'd just make sure you're propped right for your boat (low pitch) and find out what rpms you're running.

 

[steelespike]

Re: which engine for semidisplacement hull

I think you should go for the most efficient hull speed.Probably between 6 and 10 mph.It appears now it takes close to 30 hp just get that extra 2 mph.A semi displacement hull is about the only one I would consider going down with a horsepower change.The 40 big foot should do really well and you might even find a 30 or 35<br /> heavyduty pusher that would save even more gas.<br /> Its possible at 8 or 9 mph you could save as much additional as the 12 to 10 mph drop.I've seen semi displacement hulls that get 28 to 30 mpg at displacement speeds.At those speeds its easier to talk,the boat is more stable with calmer movements, you can enjoy the passing world and of course wear and tear on the motor is less