PERKINS 4-108 HP

[terry_thatcher]

I have an old 4-108, put into the boat when she was built in late 1979. But although I have owned the boat for 20 years, I have never known the Horsepower of the engine. The manuals indicate it varies from 40 to 50, depending on model or something. Does someone out there know the actual max brake HP of these engines? Thanks.

 

[dave_a]

Terry, like all engines the power varies with rpm.
If you believe the factory dyno chart, its 51hp at 4000 rpm. No sailboat ever runs close to that rpm, so this from the owners manual might be helpful. I ran my engine 22-2700 rpm. If I was in a big, big hurry 3000 rpm. It was over propped with the big 3 blade , so above 3000 it would start to show black smoke and squat the stern excessively. IIRC, the boat would achieve 6K and the engine was putting out 25 hp +/- around 2500 rpm, something like that anyway.

 

[chawakee]

Allegedly the 4-108 only made 49-50 at very high RPM's.
Realistically and in service, it was a 40 hp continuous rated engine
at 3000 RPM.
Literature I’ve read states the Perkins 4-108 50 hp at 4000 RPM.
I wouldn’t dare do such a thing!
My mechanic suggested...2600 to 2800 RPM’s would have given about
30 to 32 hp when the engine was new.
Rob McCabe.

 

[Cardo]

What is a good cruising RPM for the 4-108?
Thanks in advance,
Rich

 

[dave_a]

Depends on the prop, but I had the OEM 3 blade so:
I motor sailed at 2-2200 rpm.
Cruised at 2500-2700 did 6-7k depending on conditions.
Rarely went to 3000, too noisy and you don't make much more speed than 25-2700.
Never above 3000 except to see if it would over heat (it didn't). Max rpm was 35-3700 where it squatted and made black smoke.
YMMV, but that's my 2 cents.

 

[wild382]

We do about 6.2 knots at 18-2000 rpm. Haven’t had a chance to see what we would do in rough conditions.

 

[dave_a]

John - you have a feathering Maxprop, correct? What is your max rpm at wide open throttle? Or maybe max usable RPM and speed?

 

[Cardo]

Thanks for the replies. We have been doing about the same as John, about 6 knots at 1800 rpm.
I believe the prop to be original to the boat. 3 -blade.
I was surprised by the 2500-2700, Dave.

 

[jimcleary]

Rich
Dana has the Yanmar engine with a fixed 3 blade prop. We cruise at 2200-2300 rpm and do 6.2 knots in a calm sea and with a clean bottom.

Jim

 

[dave_a]

Cardo, we have to factor trans ratio's in as well. After 40+years these boats can be all different. I have a Hurth/ZF 150 trans which replaced the ZF100 somewhere in the boat's life. (I do not recall the gear ratio)
Also "diesel experts" say to run a diesel under load or around 70% of max rpm. 2700 is my "long way to go, short time to get there" - cruise speed. Very close to 7kn at that rpm in flat water. I like the anchor down before cocktail hour.

 

[wild382]

John - you have a feathering Maxprop, correct? What is your max rpm at wide open throttle? Or maybe max usable RPM and speed?

Yes. I think it’s 16” something too...Alan Shedd has the same one.
We have new tranny from Atlantic Diesel...along with new cutlass bearing and dripless.
We have the forward/reverse gear being used opposite .
We only motor around the lake here not too long or very far. 30 mile stretch at the most. We may change it back next season if we find that it is a problem. We do go a little faster. It is pitched correctly though according to our mechanics ...

 

[Cardo]

Thanks guys. Having your experience validate things I have read is really helpful.

 

[Richard Noddin]

Yes. I think it’s 16” something too...Alan Shedd has the same one.
We have new tranny from Atlantic Diesel...along with new cutlass bearing and dripless.
We have the forward/reverse gear being used opposite .
We only motor around the lake here not too long or very far. 30 mile stretch at the most. We may change it back next season if we find that it is a problem. We do go a little faster. It is pitched correctly though according to our mechanics ...

Hi John,
I also have the 16" model Max Prop, and still run the ol' Perkins 108. I was wondering what pitch setting you are using. I think I set mine at 8.6", but seem to be a little low on the speed (~5.8kn @2000 rpm, 2.5:1 reduction). I a precise any input. Thanks!
Rick Noddin, Xanadu

 

[wild382]

I’ll have to get out the settings from an email. I can’t remember exactly what it is. It’s not bad to be geared “a bit” lower. It’s good for heading into current, rough water, etc. What that “bit” is needs an expert. The image above is for the gearing of our new transmission.