Re power with outboards

Greetings all. I have a Ted Hill "Bluehill" semi planning boat. Currently has diesel inboard rated at 65hp @4000rpm. Ony running at around 2000rpm cruise which produces about 8mph. I plan to change things a lot and fit 4 stroke outboard/outboards. Anyone done anything similar. Would love to be able to get away with two Yamaha FT 25s and get my current cruise at 3000 rpm. Any tables out there to figure such things out? Fuel economy at this speed is my priority. At about nine she starts to squat and would need lots of power to plane out (lots of fuel). I went game fishing yesterday and ran for 10 hrs at 7 so fuel use it critical to me. Saw some great older threads on this subject. Any of you guys have updates? She is 27' 6k lbs.

 

My calculations show about 25 Shaft Hp to produce 9 mph. I would think your present set-up, 65Hp @ 4000 rpm is almost Ideal, perhaps slightly more than what's needed. Running at 2000 rpm should be economical, especially in a diesel. If you need better economy, I would consider a smaller diesel.

 

Thanks for that input. Yeah my current set up is good fuel wise just takes up tons of room in the boat and is heavy. Has a shaft to a v drive forward which whines and drives me nuts! Economy is awesome at about five litres hr for my tired nissan diesel. If I run a 25hp at 5000rpm to get 9mph It will use 9.2 litres hr. No one wants to listen to that right? So my question is what happens when I run 2x25s at 3500rpm or so (efficient rev zone) to my fuel economy and speed? I figure two would get more thrust per revs than say one 50hp? As my boat has a rounded stern she suits one engine either side and a walk thru in the centre as I dive a lot also.

 

One Engine (with Double the HP) will get about the same Thrust as two smaller engines since Prop will be larger and probably turning faster due to different gear ratio. Probably a trade - off, but in theory, one engine should be slightly more efficient due to less gear loss. Either application gives about 43% prop Slip. I believe however, that either way you go, more fuel will be used than with the diesel.

 

Forgot to mention. If your weight reduction is appreciable, 1000 lbs ? , you may be able to go down to twin 20 hp or even 15 hp and use less fuel and still maintain 8 mph at lower than full throttle.

 

Thanks again. I agree I will always spend more in fuel going to outboards. Just want to keep it within reason. How do you do your calculation?
If I go with two 25s weight is 180kgs One 60 thrust motor (yams) is 118kgs.
2x25hp fuel max is 18ltrs hr one 60hp max fuel is 21ltrs hr. One 60 would be cheaper and easier to set up and it's the next high thrust motor after the 25. My existing engine, gearbox, vdrive, shafts and steel mounts are heavy but If I shed 1000lbs I will be lucky, I guess the rudder and shaft go to and they are another big chunk so lets say 1000lbs.

 

As I indicated earlier, you may not need 50+ HP, if speed is 9mph max. 30 - 40hp running at less than full throttle should do it.
Easiest way to calculate what you need is download:

www.castlemarine.co.uk/pitch.htm

 

Thanks again for your help, much appreciated. Looked at your profile and those are some sleek craft you have!
I had a little 16' 1960s silverline with a merc sternleg that I used in lake powell a few years back. It was baby blue with blue vinal interior and chrome everywhere incl chrome glove box! It was mint and I had as much fun Looking at it as running it. Had a 64 Riviera at the same time though I didn't tow the boat with the Riv (sadly)

 

Thanks for the compliments. Actually, all three pics are the same boat, taken with different lighting conditions. Boat was custom built by myself and is very fast, 85 mph+ with 2 people onboard.

 

Had a better look after you said you built the boat. The only other engineer I know is a perfectionist also. Stunning work. I guess fuel economy was NOT a consideration in the build (we all remember those days). Been working with the calculator you supplied and it's fun. I bring my boat home each winter and this winter I will be removing the rear deck (wasted space) and building a transom door for diving and hauling those game fish aboard. I will try and borrow an outboard (tiller) and rig it on the swim platform just to get some feel for things..........."Milly" is pictured on Teds site www.tedbrewer.com/power/bluehill.htm Cheers, Garry.

 

I can see why you want to get rid of the inboard and free up some space. Real nice plan, but not alot of outside space.
Happy to hear you're making use of the Prop calculator. No fuel consumtion was not even considered. If I keep my "foot in it" for even a few minutes, I can almost see the pointer on the fuel gauge moving. Typically, I go out for an hour or two and only open her up for a very few minutes or even seconds.
You might consider trying your "outboards fixed" (not rotateable) and using your existing rudder. Also you can get quite a bit of manuverbility with Dual throttles and shifting.
By the way, you can enter my numbers into the calculator to obtain prop recommendations and speed
1.High speed runabout - 20 ft
2. 475 HP @6400 rpm
3. Displacement 2200 lbs (one person & 1/2 tank fuel)
4. 1:1 gear

Have fun, Jango