SMALL BLOCK CHEVY

[THUNDERBOLT]

HELP!!!!!
Anyone out there who can help with little problem......
THUNDERBOLT THE 1961 COWES TORQUAY WINNER is to be fitted with 2 rebuilt 5 litre Chevy small blocks totally rebuilt with flat top pistons and electronic ignition and producing approx 220/225 hp each...BUT without going to the expense of Dyno testing which I can ill afford now does anyone know what approx revs for economy these will return.....she is shaft driven and need to find best prop size.
25FT RAY HUNT DEEP V WOODEN WT APPROX 3.5 TONS

NB She originally had 2 325 hp Crusaders with shaft rpm of 3300..running 14inch diameter 17 inch pitch any old school wizards out there can help??

Mike Classic Offshore UK
OR RESPOND TO www.TOMMYSTBOLT@aol.com

 

[Griff]

Probably 3000-3500rpms for the engines would be the best efficiency range. WOT rpm should be around 4800rpms.

 

[Beeper]

Hi
I sent an email reply to www.TOMMYSTBOLT@aol.com and it did not go through. It was undeliverable. Also saw your posting about the Ghost Rider. I knew what the boat was. This boat was in a boat yard in Quincy Ma. USA many years ago. It had been donated to Boston College(?) and was being shown to a prospective buyer possibly Barry Cohen. The boat had some rot issues and read that some one was going to make a fiberglass knockoff from the original. Would love to know what followed after it left Quincy
Bill

 

[Beeper]

Hi
I read you inquiry in Seriousoffshore.com about your repowering of a classic speedboat. Fuel efficiency is not just finding where the engines run well but more important is where the hull is most efficient. You want the hull to be producing a smooth wake with the bow coming down in angle after it gets on plane. What ever this boat speed is, is where you want the engines to be near their peak torque rpm or some what below that. Typically a small block Chevy developing max power at 44-4800 rpm will develop max torque at 32-3500rpm. If you can get the hull up to a good cruising speed at 28-3200 rpm and top out at 4500rpm+then you will be doing about all you can.
The rule of thumb is that for every 100 hp a gas engine develops it will burn approximately 8 (US) gallons an hour at full throttle So two engines totaling 450 HP will burn ( 8x4.5= 36 Gal hr) at full throttle. The second rule is that engines will burn approximately 1/2 this amount at 3/4 max rpm. 4500rpm x.75 = 3375 rpm = 18 gal hr. If the hull will reach an efficient speed at less then 3375 the fuel consumption will improve noticeably. If you slow down to say 2500rpm and the boat starts to fall off plane the fuel Mileage will decrease. The gals per hour will be less then at 30-3200rpm but the speed will be much lower. The very best economy is achieved at idle speed or slightly above and the boat will running as a displacement hull. Probably about 5-7 knots.The final measurement is how far you can go on a given amount of fuel.

Now we come to the boat itself. Keep it as light as possible, make the bottom paint smooth and apply the paint in the direction the boat moves. You said the original setup turned 3300 shaft rpm from 325 Crusaders. This seems to be about a 1.5 to 1 reverse gear (or v-drive ) reduction ratio. Do you know the exact gear ratio? You wrote that the boat turned 14"dia x 17" pitch prop. Can you measure the shaft angle in relation to the boat bottom and the distance between the top of the prop and the boat bottom right above it. I assume the props are 3 blade bronze. What is the shaft diameter. Do you know what the displacement of the original engines are? Liters or cu in. How fast did the boat go in its day. Is the 14x17 prop its original size? Does the boat have stern trim tabs? Fixed or adjustable? You stated the boat weighs 3.5 tons. That seems heavy to me. You said the engines are 5 liter. Are we talking about a 305, 307 or a 302 cu in Chevrolet engines. Do the engines turn opposite rotation at the crankshaft? I would have built 350 cu in (5.7) engines for a boat this heavy. Same physical size and weight. Some parts are cheaper then the smaller engines. Are the camshaft lifters flat tappet or roller type. Self adjusting hydraulic I assume.
The more info you can give me the closer I can get to the correct prop size. Hope this gives you some direction.