Cooling Configuration on 4-108 Perkins

[Alan Wheeler]

Ooops, thanks Bill for picking up my mistake there. Yes it is 7kts.
Although two or three blade makes no actuall difference to pitch, but it does make a big difference to slipage which equates to a three blade behaving like it has more pitch when comparing to a two blade. A cupped blade is similar. The number of blades has a direct relation to friction, more blades more friction, thus more power required to turn the prop. A Slow reving prop equals more torque, so the friction is not so prevelant. Fast turning props rob the engine of Hp because it takes so much to spin them in the water. A two blade is slightly less efficient, but require less Hp to spin it. So what exactly is the right choice tends to have a major couple of fudge factors that the experts get a feel for over time.
Jim, for a displacment hull, boat weight has little to no bearing on hull speed. It does however, determin the power required of your engine. 35Hp should be adiquate although at the bottom end of the adiquate scale. This means you will have little spare power and an incorrect prop will rob you of that little power you have left very quickly.Your water line length is criticle to speed. Your shaft rotation is critical to the power required to push the boat forward. 973RPM equates to exactly a 2.57:1 ratio box. Using 3000RPM as max RPM for that wee engine (I don't know if that is rated max or not) then a pitch of 8.1 for 10% slip to a 14.7 for 50% slip is required. I would be heading toward the 10% slip end of the scale.

[jimisbell]

I keep forgetting to mention that I have a 3 bladed prop....I think.....that is what the PO reported to me when I bought the boat. I didnt ask the diver that inspected the bottom at the time of purchase in Tampa.

My Perkins manual suggests up to 4000 RPM in pleasure use and 47 HP at that RPM. I dont want to run it there.

[CSY Man]

My Perkins manual suggests up to 4000 RPM in pleasure use and 47 HP at that RPM. I dont want to run it there.

My Perkins manual list's the HP as 51 @ 4000 RPM.
That is only "emergency" power however.

Max continous is 3000 RPM, but I usually cruise @ 2100 RPM. It seems to be the bst combo of speed, fuel consumption, less drag, etc. Also, no black smoke.

[SkiprJohn]

Aloha Jim,
If you currently are using an 18x18 3 blade (we'll know soon), that almost definitely is the overheating problem. Michigan wheel suggests I use an 18x13 or 18x14 and I have a comparable engine.
Kind Regards,
JohnL

[jimisbell]

I sure hope you are right because that will be an easy fix. If that is the problem, and I am beginning to think it is in spite of my propcalc program, it will mean I have one of the best cleaned, efficient cooling systems around because of 6 months of work to make it cool better...#8-)

If that propcalc program is wrong then there are a lot of people out there that will be putting the wrong prop on their boats. According to that program I should have a 19x16 prop!!

Will a prop shop change the pitch on my prop from 18 to 13 or is that too much movement?

[SkiprJohn]

Aloha Jim,
Prop shops are pretty good at changing nearly everything. Its worth a call. They'll want to see your prop first. Your engine develops a bit more power than mine because you have a 4000 rpm max. Mine only has a 3000 rpm governed max and has 35hp at that speed but I would say they are pretty close.
I think a change in prop pitch to 14 or 15 wouldnt be too hard for them but take a look at your prop first and see what kind of condition its in. It could also be a power boat prop with wider blades. All worth looking at before decisions are made. 3 blade sailboat props come up on eBay quite frequently or are available at second hand shops. You might even consider a two blade?
Kind Regards,
JohnL

[jimisbell]

Since I am using my boat as a motordsailer and the emphasis is on MOTOR wount a 3 blade prop be more efficient??

I got the Tach working....ALMOST.

Its an alternator driven tach and it seems like that when the batteries are charged it quits reading. When the batteries are recieving charge it reads..now. I had to recalibrate it as it was not made for this alternator which apparently has fewer poles than what it was designed for. I had to go inside and short out a resistor to get the amplifier to read high enough but it now is squarely in the middle of the calibration pots range so that was a productive day. Now if I can just get it to read when the batteries are fully charged...but that fodder for another forum.

I am off to the prop shop this afternoon and see if they can repitch it for me. Even if the one on the boat is correct, I still need a spare that has the correct pitch.

[SkiprJohn]

Aloha Jim,
Do you have a motor sailer or is that just the way you use it?
Your tach problem is way beyond what I know about electronics. I can usually tell by sound how fast the engine is going. I'd like to have a tach that reads from the alternator buy my alternator only has one connection wire.
Kind Regards,
JohnL

[Alan Wheeler]

Jim, do a search and see what you come up with. That very issue was discussed here a while back. Something to do with the regulator. Rick I think gave the in's and out's about it. If you have no luck, let me know.

[CSY Man]

I run a 16 inch X 10.5 wide three blade prop on my CSY 33 with the 4-108.

If the overheating problem is related to yer prop, check the max RPM while underway: If ya get 100% of rated RPM, ya are good.
Then check it at while tied to the dock: If ya get 90% at max throttle while static, ya are good.

(Not over-propped)

Some guys run over-propped on purpose..To save fuel and to run lower RPM...Say 80% RPM at max throttle.
It may work for some, but not me...Can't afford a new engine anytime soon.

[jimisbell]

Originally Posted by SkiprJohn

Aloha Jim,
Do you have a motor sailer or is that just the way you use it?
Your tach problem is way beyond what I know about electronics. I can usually tell by sound how fast the engine is going. I'd like to have a tach that reads from the alternator buy my alternator only has one connection wire.
Kind Regards,
JohnL

Its a real motorsailer, a 1972 Gulfstar 36 Center Cockpit.

[jimisbell]

Originally Posted by CSY Man

I run a 16 inch X 10.5 wide three blade prop on my CSY 33 with the 4-108.

If the overheating problem is related to yer prop, check the max RPM while underway: If ya get 100% of rated RPM, ya are good.
Then check it at while tied to the dock: If ya get 90% at max throttle while static, ya are good.

(Not over-propped)

Some guys run over-propped on purpose..To save fuel and to run lower RPM...Say 80% RPM at max throttle.
It may work for some, but not me...Can't afford a new engine anytime soon.

I took the 18x18 prop to the shop this afternoon and he says he might be able to get it to 18x13 but he isnt sure but he will see what he can do. I think if he gets it to 18x14 I will be happy. Cost? "about $55 bucks..."

This way when it comes out of the water on Monday if its overpropped I will have one to slap on there right away and wont have to wait lay days at $10 a day till a new one is delivered.

[Alan Wheeler]

Sounds like things are on track for your Jim. I look forward to hearing the news of how it all works out.

[Andy Copeland]

I had the same problem when a rebuilt 4-108m was fitted a few years ago. Half a dozen professionals fixed everything but the problem! finally a new fresh water pump was fitted and voila! an engine that runs reliably at 180 degrees forever.
Andy Copeland

[jimisbell]

On monday I will haul out. and check the prop pitch if that is OK then the freshwatrer pump comes off!! Thanks for the comments. Up till now I had not heard of anyone having a problem with the Fresh Water pump so had put that on the back burner for last. Did it have any erosion to the impeler or were there any improper gaskets on it? Any explination as to why it wasnt working?