Diesel motor gearing?

Hi all

I have recently been intrested in building a wooden fishing boat. I like the Vee Gull from Glen-l. http://www.glen-l.com/designs/inboard/veegull.html#avail

I would like to put a 125 hp yanmar diesel motor in the boat. Glen L said this motor would give me 26-27 knts top. I was reading about reduction gears. Is there such thing as an increaseing gear ratio for a boat, as in cars? Idealy I would like to have a two speed transmission. low gear 2:1.75 and a high gear 1:1.15 or even 1:1.25. is this possible? if so why do more people not do it. I would imagine the fuel eco would go up as would top speed. if this would work my top speed would go up from 26-27 kts to 30-31 a good speed boost.

thx alex

 

I would think the speed would be limited by the HP of the engine and gearing with the right prop wouldn't make a lot of difference. My boat has 120 hp diesel with a 2:1 outdrive. When they tried using 1.7 drives they had problems with the engines over heating. Apparently the gearing does affect load on the engine. maybe they weren't compensating with the prop? When they went to 2:1 gears the engines lasted longer? There have been some "swich pitch" props that have been tried but I know absolutely nothing about their performance. A 2-speed trans like an old powerglide does sound interesting. I think they did use multispeed transmissions in some drag boats. No clue if they still run them like that.

My boat is 26.5' and runs mid 30's (mph) with the 120hp mercruiser diesel.. It also gets 6 mpg at cruise in the upper to mid 20's. You'll love the economy of the diesel once the engine is paid for. The mercruiser 1.7 is also weighs 650lbs with an alpha outdrive. Not sure waht a bobtail weighs..

 

"I would like to put a 125 hp yanmar diesel motor in the boat."

125 hp is 125 hp, the best a transmission can do is allow a useable propellor that will efficentluy absorb that power.

The usual marine setup has almost 100 years of practical knowledge behind it, and works.

FAST FRED

 

ZF Marine Gears makes several models of two-speed marine transmissions, but none for as low a horsepower rating as 125 hp. I believe that the ZF 90TS is rated for between 175 and 290 hp with gear ratios of 0.97:1 to 2.79:1. This is relatively new technology, so the longevity of the device is somewhat unknown.

 

Keep in mind that torque turns the propeller and torque is inversely proportional to RPM. As your gear ratio goes up, torque at the prop goes down.

In english units: Torque = 5252(HP) / RPM

In a fairly high speed engine, which most Yanmar's are, you need to look at the torque curve of the engine and the torque at the prop. This is why you will find lower ratios on these engines. You can easily get to the point where you don't have the torque to turn the prop as you would expect. You may end up with a boat that will not accelerate very well.

Obviously the form of the boat plays into all this (hull form, displacement, trim, etc. etc.)