Cape Dory 25d acceleration question

[AMcC]

I became the new owner of a beautiful Cape Dory 25d this summer. The prop has been replaced with a three-bladed prop. The boat seems to have a problem where if you ramp up the throttle too quickly (anything other than very slowly), the prop seems to spin away and the boat never gets up over a knot and a half or so. If I bring the revs up very slowly, the prop seems to 'catch', and it will accelerate and cruise pretty at a decent speed (for a boat of this weight with this small an engine Y1GM). Does this sound normal? My only real concern with this is that if I needed power more quickly, it simply wouldn't be there... Possible issues if not normal?
-Alec

[Jim Walsh]

Absolutely not normal. Perhaps someone who has a 25D will chime in with their experience with a three bladed prop and the size so you will be able to compare it to the size of your prop. Hopefully you don't have a transmission which is slipping. Have you checked the tranny fluid level?

[steveg]

My experience is with a two bladed prop. It does not accelerate rapidly, although it is adequate. It does not feel like the propeller is slipping. If heavy throttle is applied it accelerates more quickly.

What you are describing does not sound normal. It would be good, as suggested, to check the trans fluid. You can also look over the engine, a little tricky, to see if the shaft is spinning when you are in gear.

[Joe Myerson]

I've got a newish three-bladed prop and shaft, which I had to replace after an unfortunate run-in with a winter stick. I had the prop matched to my 1 GM, which the original one did not seem to be.

That said, when I bought my boat, the PO advised me never to accelerate quickly, and I've always followed his advice, easing the throttle lever forward until I reach the desired speed. It seems to work.

Still, the prop's spinning does not seem to be normal.

Good luck!
--Joe

[RC James]

Currently running a 12X9X1 3-blade on my 25D for the last 10 years, replacing the 2Blade 12X12X1.
The 3 Blace came w/the boat as a 12X12X1(loose in the port locker), and I believe JohnVigor had recommended a 9" pitch.
Works well at 12X9, and will speed up readily, but I don't HotRod the throttle. I use a steady increase in throttle as the boat picks up speed.
RC

[Joe CD MS 300]

Sounds like what you are expierencing may be what is commonly called “cavitation” . Not sure if that is the correct term it may actually be “ventilation”. It occurs with a prop that is pitched too much for top end speed rather than low end torque.

Cavitation plates are often added to outboards to help prevent this I think by increasing the pressure around the prop. This allows the prop pitch to be optimized for top end speed with less potential for low end or starting cavitation.

Any prop shops in your area? I only know enough to know I don’t know enough.

[Frenchy]

I'd suggest what two posters have already mentioned: check the transmission fluid. If the transmission is over-filled, it
will result in slippage. I learned that the usual, hard way. - Jean

[Steve Laume]

I seriously doubt it has anything to do with the prop.

Raven was experiencing slippage in the transmission a few years back. Nothing had been changed as far as the prop was concerned and I never had any prior problems. It was most notable when in forward gear. She has a Universal engine with a Hurth transmission. When at the Wooden Boat Show at Mystic that year, I asked the Universal rep about it. Mostly whether it would last the season so that I could deal with it in the fall. Fluid levels had been checked and they were normal. He advised me to change the fluid and that I should be able to make it through the summer if I babied it a bit. I did as he recommended and then pulled the engine and had the transmission rebuilt that winter. End of problem.

I know this is not what you wanted to hear but may well be the sad fact. There isn't much fluid in there to begin with so it should be carefully monitored and changed often. The manual on my transmission also states that it should be locked in reverse gear when sailing.

The only other thing you might check after changing the fluid, is the shift linkage. It should be crisp and complete. You could eliminate the possibility of the trans not fully engaging by having two people on board and disconnecting the linkage at the transmission and then having one person shift it at the transmission. All it would take is a slight shift in the holding clamp for the cable to create a slippage problem due to not having the transmission completely engaged.

I hope you discover something simple but I would not be looking at the prop, Steve.

[Joe Myerson]

The only other thing you might check after changing the fluid, is the shift linkage. It should be crisp and complete. You could eliminate the possibility of the trans not fully engaging by having two people on board and disconnecting the linkage at the transmission and then having one person shift it at the transmission. All it would take is a slight shift in the holding clamp for the cable to create a slippage problem due to not having the transmission completely engaged.

Steve makes a good point, which I'm embarrassed to say I forgot.
My shift linkage got seriously corroded and eventually snapped. Before it broke down completely, it became more and more difficult to shift.
But you have not mentioned any sticky shifting, so it might not be the linkage.

However, your throttle/shift lever mechanism is pretty exposed, on the inside of the port cockpit locker, so be sure to keep an eye on it.

Good luck,

--Joe

[sgbernd]

You probably already did this, but to state the obvious....

Any chance the transmission has the wrong (too heavy) lubricant? For example, my Volvo MS-10 used 30W oil and the Beta uses ATF. My friends V-drive uses gear oil. If your Yanmar needs ATF but somebody put in SAE30 or 90W gear oil, I could imagine it having the effect you describe.

When the clutch is slipping, usually they start to slip under high power. The amount of power you can put thru the transmission before it slips decreases as the wear progresses. But you describe it holding if you "sneak up on it" which indicates the clutch is able to hold at full power. If the lubricant were too heavy, I could imagine the clutch having trouble grabbing, particularly immediately after engaging gear. But after it settles in, it holds.

Confirm the lubricant currently in there is correct for your transmission. ATF is light and dark red colour, SAE30 is golden brown motor oil, Gear Oil is heavy, brown, and smells like rotten egg.

If you have the correct lubricant, unfortunately, you are probably in for transmission work.

Steve Bernd

[AMcC]

Thanks to all for your suggestions - and no, I haven't checked the trans fluid level or what kind of fluid it might be yet... I did take a good look at the shift mechanism in the port lazarette, and that seemed to work cleanly. This gives me more to check. The boat came out of the water today for winter storage. They guy I bought it from was pretty fastidious about everything, but I'm glad to know I'm not crazy and that there is likely a solution out there to getting more power out of the engine/prop in a more 'on-demand' fashion than I am currently getting. It's the only significant issue I have with the boat so far.
-Alec

[Joe CD MS 300]

That’s interesting. I was not aware that a transmission without a clutch would slip. Thought it was either go or no go.