Correct prop pitch

[john1970]

I have a 1992 Chaparral 1800SLC with a 4.3 volvo penta and a DP-c duo prop. My problem is that the engine rpm's starts to rise over 4800 and i have to throttle back a bit to keep it in the safe range. With a full load (full tank of gas and 4-5 people) it will top out at 4800 rpm's at 50mph but whe the fuel level starts to drop the engine rpm's start going up and mph increase to 53 to 55. I run b7 props now. I wonder if b8 props would be too much? Anybody have this issue?

 

[Silvertip]

Re: Correct prop pitch

Whether you switch props or "should" you switch props depends on which load you carry. If you run heavy most of the time, stay with what you have. If you run light most of the time go with more pitch. But remember, if you go with more pitch and then try to run heavy you begin seriously lugging the engine and that means across the entire RPM band. Overreving you can always throttle back. Underreving or lugging means you cannot throttle up.

 

[Robbabob]

Re: Correct prop pitch

Seems to me, if all you do is spend fuel and the rmp start going over, then it's time to add pitch. My guess is, when you run a lighter load, the rmp are already wanting to go over....

Roob luck!


just a small accomplishment of meeting a 300 post level! :?}{:|} just numbers... \/ means nothing more than the iformation i've been giving :facepalm:

 

[Silvertip]

Re: Correct prop pitch

RPM does not cost you fuel -- "load does". The "amount" of throttle opening to achieve that rpm costs fuel. The heavier the load (weight or loading due to prop pitch) the more throttle is needed to achieve the same speed. While this is not recommended, put your engine on muffs in the driveway and rev the engine. That engine will hit 5000 rpm before it ever gets anywhere near WOT. Why -- "no load". Ever notice your car loses speed going up a hill and the cruise control (or your right foot) needs to add fuel to maintain the same speed. The same principle applies in a boat. However, unlike a car, boats don't have transmissions so they can't shift down when load overcomes available power. If prop selection is wrong at the upper end of the rpm band it is wrong across the entire rpm band. A worst case example would be using your existing boat and adding a prop with about 4 - 6 inches more pitch to the prop. Engine rpm would drop tremendously, you would not be able to get on plane and fuel consumption would be through the roof because you would need the throttle wide open to get anywhere. That is why the manufacturers want their engines propped at WOT. It is not because the engine will fly apart if it goes faster than that. It is because that is where they want the engine to run.

 

[Hydro Head]

Re: Correct prop pitch

RPM does not cost you fuel -- "load does". The "amount" of throttle opening to achieve that rpm costs fuel. The heavier the load (weight or loading due to prop pitch) the more throttle is needed to achieve the same speed.

I'm not sure I agree with this. More prop might mean more throttle opening because of increased loading at a given RPM range, but not necessarily more fuel.

Think about it. Why do cars have overdrive. If you put more prop on your boat it is like overdrive in a car. A car has gears that help with acceleration and a boat doesn't, but once at cruise the analagy is exactly the same. More prop should result in better efficiency at the expense of acceleration.

A gas engine has a throttle. The throttle is used to restrict the amount of air allowed into the engine to control power output. This is actually a detriment to efficiency and adds to "Pumping Losses". An engine is like an air pump and very inefficient at small throttle openings because it expends a great deal of energy just trying to pull air in. This is perhaps another reason why Diesel Engines are more efficient, they don't have a throttle.

Decreasing the gear ratio (overdrive) or adding more prop pitch results in less RPM and requires greater throttle opening to produce more torque to achieve the same amount of net power. Technically the amount of air and fuel that it brings in will be less because again RPM is reduced and the engine is spending less energy pulling the air in. A gas engine is at it's most efficient when geared to achieve the largest possible throttle opening. Unfortunately this would put tremendous stresses on internal parts(crankshaft, bearings, con rods, etc.) and is why Engine Manufacturers give recommended minimum RPM at Wide Open Throttle. So there are reasonable limits to this theory.

I really believe that if you are an open water cruiser and are willing to give up some holeshot you will see better fuel economy with a larger pitch prop. Again think overdrive.

 

[SparkieBoat]

Re: Correct prop pitch

I think you are correct..It will depend on how he uses the boat. If the boat is reaching the limited RPM range when loaded I would go up enough to keep it under the max desired RPMs. I generally pitch my boats so they reach max RPMs unloaded..but thats just me.

 

[john1970]

Re: Correct prop pitch

I'd like to thank everyone for their thoughts on this matter. I was told today that a B7 prop has a 26 pitch (kinda high i thought) but with a 2:30:1 gear ratio in the lower unit i can see it work. As for the hole shot it jumps out of the water like a fish getting chased..lol but for the size of the boat i think the outdrive is a bit overkill and most small boats like mine come with a 3 liter with a single prop outdrive. I get alittle prop burn on the aft prop but i was told that was normal due to the way the outdrive was designed. I think who ever matched up the prop setup with the boat did a good job for the most part even though i can go over the 4800 mark by 2 to 3 hundred rpm's which i don't think is that much but who am i to say that lol. It's a toss up if i go to a set of b8 props because my operating range is between 4400 and 4800 rpm's, expensive gamble if they have much pitch.

 

[Silvertip]

Re: Correct prop pitch

Overdrive is a very good example of why you are both wrong. It takes about 40 HP to push a car along a flat highway. What happens when you approach a hill? Yup out of overdrive and if its steep enough the converter clutch releases as well. A boat takes probably 1/2 of its available power just to get on plane so overpropping it can decrease fuel consumption only IF -- repeat IF -- the boat is severely under propped to start with. Lets get back to the overdrive situation. Overdrive drops engine RPM 18% - 20% depending on vehicle. If you overpropped a boat by 18% you would burn much more fuel getting on plane because it takes more throttle, and more throttle longer. Drag at cruise is a constant regardless of prop so one would think that doesn't factor into this disucussion. Wrong again! IF -- repeat IF -- and only IF the engine has enough power to actually twist that 18% overpropping would make this less fuel discussion hold true. Fact is, if you overprop by 18% from the manufacturers suggested WOT rpm your boat is very simply propped wrong and I would wager you see a fuel economy drop as a result. You can't stick an overly high pitch prop on an already properly pitched boat and gain economy. Physics 101. Your thoughts were true, every boat would have 26 pitch prop regardless of engine size -- and most of them would need a tow to get on plane. Final test for you. Next time you are in your car and sitting at a stop light -- put the gear selector in second gear (not drive or OD). Most automatic transmissions will start in second geat. Now accelerate. If what you say is true, why is the car a dog vs starting out in 1st gear. It is the "load" that results in you having to have a pretty heavy foot to overcome that load. That burns fuel. Wide upen throttle under heavy load does not guarantee high rpm because the engine has run out of power. Only if you keep that engine propped so it is in its power bank can it operated most efficiently. Underprop and you burn fuel by excessive reving (overreving the upper end of the band). Overpropping lugs the engine so you again loose economy due to lugging and resulting increased throttle to maintain the same speed.

 

[SparkieBoat]

Re: Correct prop pitch

best if you can borrow a set of props maybe 2 pitchs higher (should lower your max RPM by about 300-400 and you should gain 2-4 MPH) and do a test..they are pricey. maybe a boat shop would have a used pair they could loan you with a deposit. I do not think a 2 pitch higher prop would cause the engine to lug. But I do agree with Silver..it is better to back off a little in the throttle than to over prop.

 

[donnymac]

Re: Correct prop pitch

Winter time when it's just me a six pack and some fishing rods I run a 19. She will barely pull 4700, close enough! Summer time with kids innertubes etc. I'll run a 17. Still only pull 4700. Still close enough. When I take her out in the summer time by myself(whee!) it will pull 5200. Too much, throttle back. I suppose it doesen't matter because I cruise at 3200 anyway, but food for thought.