Prop selection for economy

[kkboat]

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.

[jimisbell]

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.

[kkboat]

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.

[jimisbell]

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

[oktay Çemberci]

Originally Posted by jimisbell

I have been using several prop sizing programs found on several different sites on the internet and none seem to address TWO (2) questions I have.

First the details: I have a propellor on my Gulfstar 36 (20,000 lbs) that is an 18/18 and it is driven by a Perkins 4-108 (47hp at 4000 and 37hp at 3000) through a 2.57:1 Velvet Drive gearbox. I cannot achieve anywhere near hull speed and cant even achieve 7 Knots which is what I think the economical cruising speed under power would be. When I get it up to 6.5 it begins to overheat. (I have checked water flow and replaced the raw water impeller. The fresh water is suffient in amount and does not vary in level over long periods of time..50 hours or more). There is NO black smoke and the engine runs smoothly. I just completed a 35 day, 1400 mile trip with it running at 5.5 knots at 200 degrees F and averaging 0.671 GPH.

The questions I have are as follows:

1) according to several of the prop sizing programs I could increase the diameter of the prop to as much as 23x20 and reduce the RPM to as little as 2000 and then have a slower running engine but still have the HP to push it to 7 knots. BUT....none of these programs let me enter the prop specs I now have just to check the program and to determine if my performance is any where near what the prop should be giving me. (there could be something else that I should be looking at) I think I would prefer a less radical change, perhaps to a 20x18 and reduce RPM to 2500. (2500 rpm, 996 prop rpm, 35HP about 75% of full power and 7 knots) Wheather reducing the RPM would result in a cooler engine or not is also a question since it is still doing the same amount of work, just as a slower pace. One program suggests that the optimum diameter is 20" for this combination but it doesnt even suggest the pitch and doesnt say how it comes to that conclusion.

2.) None of the programs seem to make a distinction between sizing for economy and sizing for speed. And most seem to be designed for speed in that their language suggests they are refering to larger power boats and sometimes have an input for hull shape. I am only interested in displacement speeds. To further complicate the problem the Gulfstar 36 uses the same hull for the MS as for the Trawler so its not really a sailing hull.

I am fully confused now and would like some input on this.

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

[jimisbell]

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??

[oktay Çemberci]

Originally Posted by jimisbell

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??

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