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**Inlet Marine Services** · 18-03-2008, 14:52

A 15" prop looks like it would work good the a 2.6:1 trans which is an option with the 2YM20 Yanmar. E-mail me and I will send you the calc sheet I could not attach it to this post. inletmarineservices@comcast.net The new engine might be shorter, we just pulled a 2qm15 out and droped in a 2YM15 and it was 4" shorter. The 3YM20 with trans is 27.3"

never monday · 18-03-2008, 15:23

A 3YM is about 6" longer than a 2GM. 2QM is way different.

Whimsical · 19-03-2008, 01:13

If you let the prop move 2" further back will the increase in unsupported length be a problem for the shaft?

Mike

Northeaster · 19-03-2008, 04:31

Mike - Right now there is no unsupported shaft, as the prop hub is about 1/2" away form the strut. Therefore there would be around 2 or 2 1/2" of unsupported shaft, if I move it back. I don't have any experience in this, but that doesn't seem excessive to me.

**GordMay** · 19-03-2008, 05:01

From ABYC Section P-6 PROPELLER SHAFTING SYSTEMS
https://www.abycinc.org/committees/P-06.pdf

6.5.5.4 The distance between the forward end of the propeller hub and the aft end of the last strut bearing shall be limited to one shaft diameter.

Northeaster · 19-03-2008, 05:10

Thanks Gord!!

I would not have known that. If I repower, I will have the shaft shortened, to keep within the recommendations (1" for me, as that's the diameter of the existing prop shaft).

Northeaster · 19-03-2008, 06:12

Whether I rebuild the current 2GM (13hp), or repower to something around 20hp, I would eventually like to get a lower drag, 2 or 3 blade folding or feathering prop. I have read most threads on avaiable choices.

my question, if I currently have a 3 blade fixed 15 x 10, which s probably over propped, what are the implications of going to a 2 blade vs 3 blade folding / feathering? For example, if I stay with the smaller engine, should I consider a 3 blade (maybe 14"), to have power in the chop / headwinds? If I go to a 20hp engine, shoudl I then consider a 2 blade, as I have lots of power. or am I way off, in my thinking??

MitchM · 19-03-2008, 07:27

Hi Northeaster,
I know that I used the Michigan Wheel prop calculator then double checked it with the Yanmar prop calculator... I think i ahve a file somewhere with all the gory details of those calcs in it and will try to find it for you. after sizing the new prop, we measured what tip clearance we would have, then we had the tip clearance issue resolved, by cutting about 1/2 inch of fiberglas out. we really didn't want the exorbitant amount of work in moving the P bracket. i do know that we also had to order a new prop shaft because the new Yan was a larger diameter than the old farymann. the old seafarer, loaded with all the gear and stuff was probably coming in at 11,000#. i found the Yan2GM to be fine for any conditions i would encounter on the great lakes (non tidal). however if you will be going thru the aby of fundy or hells Gate or the races on the rivers dumping into the atlantic ocean, i'd go for the larger engine if you can fit it in without messing with the P bracket... if the larger engine requires a 6 inch longer prop shaft you may have trouble fitting the prop shaft in to the space available.

MitchM · 19-03-2008, 07:35

"Quick question - On my old 2GM, I do not have a waterlift muffer - all that I have is a small rubber silencer, in the middle of the exhaust hose. Then I have a high loop in the hose, before it exits downward, through the hull, just above the waterline - not out the back of the transom. Would this setup be OK, or should I look at a new waterlift??...'

Northeaster, i think it is always wise to go with the waterlift muffler and a high and proper swan's neck in the exhaust. this solves 2 problems -- (1) avoiding the possibility of a backflow of h 2 o in a heavy boarding sea that could force h 2 0 back thru the exhaust system and into the exhaust valves with disastrous consequences--(2) the 'muffling' capability of the water lift muffler.

while you are at it put a drivesaver coupling on your propshaft to save your gearbox.

be careful on sizing the new larger engine, sounds like propshaft length could be your limiting factor and you definitely do not want to get into moving that P bracket .

Northeaster · 19-03-2008, 08:07

Mitch - Thanks for the detailed responses. I do agree, that I want to avoid any chance of backflooding the engine with water. However, do you think that, because the exhaust / water exits down, through the hull, (just above waterline) this already reduces the chances. I would think that exiting out the transom would be more at risk of backflooding from a following sea. My setup is not ideal, as you can't see water exiting - you can sort of hear it, but it is hard to tell.
If I repower, I will go with a high exhaust elbow, and waterlift. If I keep the current engine, it has the lower exhaust elbow, and I don't think I have the height required to go down to a waterlift. My cheap muffler lies flat, so it currently goes down to it.