Prop selection for economy

The Perkins 4-107 & 4-108 was designed to run at an operating temperature of 150* to 200* F (65-93*C) as so stated in my 1973 Perkins Engine Handbook. So Jimisbell, it would seem that your engine temperature is at the high end of the acceptable temperature limit. I strongly suggest you install a 160* thermostate in your engine fresh water cooling system. High water temperature (overhating) can also be caused form a leaking head gasket. The exhaust gasses leak into the cooling passage heating the antifreeze. This is bad too. You trying to push your boat to 7 kts can be achieved, but is not practical as that is not the hulls most economical design pirameter. At 7 kts the amount of water the hull pushes aside and the amount of HP & fuel burn it takes is increased by about 40%. The boat was really designed to cruise at 5-6 kts. Over proping will allow the engine to turn at a slower RPM with a moderate increase in hull speed, but when you use full throttle, the propeller will start to cavitate, the engine will most likely start to strain to reach 3600 RPM, a thick black smoke (unburnt fuel) will exit the exhaust and the engine will overheat. I have a 1975 Gulfstar 41 with the Perkins 4-108. If you have a 2-blade propeller, installing a 3-blade will push the hull a bit faster. My boat has a 15" x 12" 3-bladed prop and she has achieved 7 kts under power. The boat sold new was fitted with a 2-blade propeller.

 

I have been chasing the "overheat" problem for over a month. There appeared to be plenty of sea water coming out the exhaust so I looked elsewhere. First I replaced the thermostat...no help and I didnt expect that to help as I checked the old one on the stove and it worked correctly. Then moving on to the sea water side I removed and rodded out the heat exchanger which had half the tubes clogged....no help even though I expected it to be the cure considering the extent of the blockage. Then yesterday I looked at the sea water strainer, It had some grass in it but not enough to be a problem. But in looking closely at the cover I didsovered that the gasket was deteriorating and it was apparently leaking. Because the top of the strainer is above the waterline there was no evidence of a water leak but it appears it was leaking AIR and reducing the amount of sea water being sucked in. I havent finished testing, but it appears this was the problem as there is A LOT more water coming out the exhaust now. Today I will take it out for a run and see what happens.

Apparently judging the amount of sea water coming out the exhaust, being a subjective judgement, is not easy. Even my mechanic in Tampa pronounced it "Plenty of water" before I left on the trip.

 

Jimisbell,

The orriginal propeller pitch was 16" x 14" 2-blade. Five years ago when I bought the boat, the 3-blade prop was installed, so i have no data on the preformance of the 2-blade prop.

 

Where in Texas are you located? I am just north of Corpus Christi.

 

Propeller Size

Propeller Calculation

Dear Sir
No Problem
If you sent me the details of your boat I will calculate to find correct
size of propeller
www.cembercidenizcilik.com
Oktay Çemberci
İstanbul/turkey

 

I think I have already solved the problem with an 18x13 prop.

But I would be interested in your findings.

The water line length is 32'
beam is 12'
draft 3.5'
displacement is 18,000 pounds
engine is Perkins 4-108, 48 HP at 4000 RPM
transmission is 2.57:1 ratio

Do you need anything else??

 

Propeller Size

Dear
More details
* Drawing details (If available technical)
* Side's pictures
* Vessel Type
* Maximum available room for the propeller

Waiting for it
Oktay Çemberci
istanbul/Turkey