Need to know transmission ratio for new prop or i sell

[Maldonia]

Hey

I have a 1984 Hunter 27, with a Yanmar 1 GM 3900 rpm engine with a KM-2-A
transmission.

I need the transmission ratio so i can get a new prop that fits my boat.

Im losing 1000 rpm with the 3 blade 11 thats on it now and the engine bogs down and loses power in 35 knt 2ft chop head wind.

Ive tried everything i can think of.....Ive contacted Yanmar and hunter directly and several dealers, several prop shops,
read the forums and all i get is maybe this maybe that.

Ive been all over the engine the only id info on the trans is KM-2-A

I Cant put another 5k into this boat for a kicker to give me the freedom and safety i desire for cruising.

if this don't work I'm asking 12k for the boat........ its make or break.......

 

[Don12364]

Looks as if someone has removed the ID plate from your gearbox, though I can think of no reason for doing this.
I just Googled "Yanmar 1GM + KM-2-A Gearbox and got this:-
http://forums.hunter.sailboatowners.com/newreply.php?do=postreply&t=119471
On the first page I see the only ratios are 2.21:1; 2.63:1 & 3.22:1.
No you only need to determine which it is.
Also it does not look as if the cranking handle is a direct 1:1 drive on the crankshaft.
So:-
Operate engine STOP control and leave it set.
Engage forward gear.
Insert starting handle and pull engine slowly until at top of compression stroke.
Mark a point on the gearbox coupling (adjacent to something stationary) with white tape.
Continue to turn engine over slowly counting the number of compressions you go through whilst at the same time have a helper count the number of turns made by the coupling. Note compressions not handle turns and don't use the decompressor! You may be able to see the coupling whilst slowly tuning the handle in your installation.
You are trying to get the coupling to the mark when the piston is at TDC.

If the coupling makes 3 turns with 4 compressions the gearbox ratio is 2.63.
If the coupling makes 8 turns with 13 compressions the gearbox ratio is 3.22.
If the coupling makes 9 turns with 10 compressions the gearbox ratio is 2.21.

See attached spreadsheet for calculations.
Sorry this board cannot accept spreadsheets, doc files or anything I can easily convert it into.

 

[Honeyman]

The 1GM Yanmar was never known for its power. Your 27 probably is as heavy or more so than the 87 28.5 Hunter I used to own and it had the 2GM in it. I don't know if just a blade change will do it for you...sorry for no specifics.

 

[dakno]

hi - I have a 1981 h27 thats all origional, my father in law bought it new. It has the 1GM with the KM5 or KMS kanzaki clutch/transmission. hunter used several combos of engine/transmissions from 1977 to 1984 in the 27. they were all 1gm yanmars. There is no way this engine can handle a 3 blade prop. You have to go to a 2 with proper pitch and diameter. Also, I would be very surprised if this engine could push this boat(8000#'s) with fluids...ive had the loaded boat weighed...in a 27 knot headwind at more than 2 kts.. At 100% power(not recomended) the boat may do 5.5 in calm seas. I have found this boat will run all day at 2/3 power, at 4 kts. The engine, if run for long periods at higher speeds, will belch smoke and oily discharge.

 

[Steve Dion]

Dakno: I do not believe that you are correct. You need a prop that is matched to the engine. We just got done replacing a 2 bladed prop with a 3 bladed Campbell and that boat has picked up 2-4 kts in rough water against the current. The vessel was a Hunter 30 with the 12 HP 1 cyl engine.

I suggest that ANYONE that is having these types of issues with a their vessel look into the Campbell. These engines will NOT reach 3900 rpm's if they are properly fitted. I think they are rated for 3600 rpm MAX. If you are reaching 3900 you are under-propped.