Proper Prop Selection

[ss302]

I recently purchased a 1978 Bayliner, Saratoga, 25 foot, with a retro fit diesel engine from a
Chevrolet Suburban. The Outdrive is a Volvo Penta 280 that has a short hub, left twist, 3 blade prop with 14" dia. and 19" pitch. I can only get 9+ knots from this setup at 3,100 RPM. Does anyone have a prop recommendation? Should I go with a 16 X 21 to get more power from my engine? Please help.

 

[SkiDad]

Re: Proper Prop Selection

I'm not familiar with that engine or rpm range but you need to go with a smaller prop pitch not bigger - probably at least 16-17 pitch to get on plane - sounds like you are not getting on plane.

 

[Maclin]

Re: Proper Prop Selection

What engine was replaced? Do you know the drive ratio? What year of Suburban is the new engine from, do you know the recommended top RPM for the new engine?

As mentioned, a lower pitch prop will allow the engine to rev higher and not lug. The prop that lets the engine hit top RPM under load is the ideal pitch and will allow best top speed and best overall performance.

Also with that large of a boat with a single prop a 4 blade for more grip in the water may be the way to go if one is available in the pitch you need. More ideal would be a DuoProp lower for even more bite.

 

[MikDee]

Re: Proper Prop Selection

You need to find out what is the top rpm range your diesel engine can reach? 3100rpm sounds really low? Most gasoline marine engines run between 4200-4600 top rpm. Then you prop to be in that range. Dropping the pitch on your prop will raise your rpm, & power, but not necessarily your speed. Diameter of your prop usually is a formula automatically determined by your pitch for most boats, unless your boat is extra heavy, then larger diameter props are available, but they are not the norm.

 

[steelespike]

Re: Proper Prop Selection

There are 11 different models of the Saratoga. 5 of which use 130 or 140 hp gas and desiel motors Some weighing up to 4900 lbs.
I wouldn't figure any of them would be very fast.
We need the information requested in "read this first" just above your post heading.
I would think that at 3100 a diesel would be about maxed out. Is your speed by gps.
Is the Saratoga considered a planing hull?
Your gear ratio may be on the leg, stamped or a tag.
Using a number of ratios I get a very high slip number like 60% While a 25 footer would tend to have higher slip numbers
60% is off the chart
What size motor was replaced and is it the same leg?

 

[jestor68]

Re: Proper Prop Selection

I recently purchased a 1978 Bayliner, Saratoga, 25 foot, with a retro fit diesel engine from a
Chevrolet Suburban. The Outdrive is a Volvo Penta 280 that has a short hub, left twist, 3 blade prop with 14" dia. and 19" pitch. I can only get 9+ knots from this setup at 3,100 RPM. Does anyone have a prop recommendation? Should I go with a 16 X 21 to get more power from my engine? Please help.

I'm not sure where you plan to get a 16" diameter prop for a VP280 short hub; as Volvo only offers up to 15" dia for that drive.

You actually need a long hub drive to be able to use the big dia props(16") suitable for a diesel.

Regardless, it appears that a 15 X 17 or 15 X 15 would be better suited to your application. The GM V-8 diesels can be run up to about 3500 rpm, although they achieve their max power at 3000-3200 rpm, and their max torque at 1200-1600 rpm.

If only right hand props are available, it's easy to switch the drive to right hand by moving the shift rod on the drive.

 

[ss302]

Re: Proper Prop Selection

Thanks for the reply. You are right about not getting on step. The boat seems to plane for a short distance and then settles back down. I think the added weight of the diesel and wrong prop have something to do with problem. I do not understand why a lower pitch. I am getting about all the RPM that a this diesel engine should have. The Saratoga, with fly bridge and gas engine, that I have was about 4,300 when new, before all the previous owners additions and changes. My GPS readings for to speed are very low, as the boat should cruise between 14-17 knots.

 

[ss302]

Re: Proper Prop Selection

The largest diameter prop that I have found for a short hub, LH, is 15 3/10's. What would it take to change to a long hub? How would that help? I have achieved 3,250 rpm and that it near top range for this engine. I have thought about a double prop, counter rotating setup, but know nothing about the expense of doing that. Would I have to change my outdrive?

 

[jestor68]

Re: Proper Prop Selection

According to the prop professors, an inch of diameter is roughly equivalent to 2-3 inches of pitch.

The selection of diameter(limited by what's available) is based on prop shaft speed; which is rather low in your case with the diesel running 1000+ rpm slower than a gas motor.

The small diameter prop simply cannot generate enough thrust to push that weight up onto plane and keep it there at that low shaft speed. Larger diameter prop (more blade area) produces more thrust at lower shaft speed, which your situation.

 

[ss302]

Re: Proper Prop Selection

According to the prop professors, an inch of diameter is roughly equivalent to 2-3 inches of pitch.

The selection of diameter(limited by what's available) is based on prop shaft speed; which is rather low in your case with the diesel running 1000+ rpm slower than a gas motor.

The small diameter prop simply cannot generate enough thrust to push that weight up onto plane and keep it there at that low shaft speed. Larger diameter prop (more blade area) produces more thrust at lower shaft speed, which your situation.

Looks like I can only get a prop a little over one inch larger. So, will going to a higher pitch give me more thrust to get more power out of the engine? I feel confident that the engine will handle additional load with little or no drop in RPM. I ran this many hours last summer and it was obvious that the prop is just not putting out the thrust needed to keep the boat up on step. What pitch should I go with? Thanks again for the help.

 

[steelespike]

Re: Proper Prop Selection

Thanks for the reply. You are right about not getting on step. The boat seems to plane for a short distance and then settles back down. I think the added weight of the diesel and wrong prop have something to do with problem. I do not understand why a lower pitch. I am getting about all the RPM that a this diesel engine should have. The Saratoga, with fly bridge and gas engine, that I have was about 4,300 when new, before all the previous owners additions and changes. My GPS readings for to speed are very low, as the boat should cruise between 14-17 knots.

You still haven't told us what hp/displacement diesel. Is 3100 your max rated rpm?What hp/displacement gas engine cruised at 14-17
Is that knots? If 3250 is your max rpm then Your at 1100 rpm less from the gas engine. Thats a lot of speed and at that rpm slip will be higher further complicating the speed issue.The gear ratio may be wrong as well.
If you change to a "right twist" does it give you any better selection?A prop with a lot of cup may help with the slip
You may want to contact Volvo about what is required for going to diesel.

 

[ss302]

Re: Proper Prop Selection

You still haven't told us what hp/displacement diesel. Is 3100 your max rated rpm?What hp/displacement gas engine cruised at 14-17
Is that knots? If 3250 is your max rpm then Your at 1100 rpm less from the gas engine. Thats a lot of speed and at that rpm slip will be higher further complicating the speed issue.The gear ratio may be wrong as well.
If you change to a "right twist" does it give you any better selection?A prop with a lot of cup may help with the slip
You may want to contact Volvo about what is required for going to diesel.

The best info I can find is that this engine when new in an automobile was rated at 3,400 max rpm. Do not know what the prior engine was since there was more than one option for this model boat. I may have the wrong gear ratio as you stated but would like to try a different prop before going to another lower unit. I do not know what current gear ratio is or how to determine.

Changing to a right twist does not give me any other options that I can find.

If I do change to another prop what I'd like to know is which pitch will give me less slip? Is going to a smaller pitch going to give me less or more pitch????? I seem to have plenty of engine power just not getting the thrust I need.

Help please!!

 

[steelespike]

Re: Proper Prop Selection

At 60% slip I think you need to verify the tach is accurate. If it is reading high that would mess up our figures.
As I suggested the gear ratio may be on the leg.Stamped or a tag. It's a pain but you could count the engine revolutions to one revolution of the prop by turning it by hand. Don't know how you would release the compression to turn it by hand.
You need to mark the flywheel and prop for accurate measurement.
The stock automotive diesel may not be an ideal marine engine source.
If the numbers turn out to be accurate a 4 blade prop may help if a large diameter prop won't fit.
We can't make an informed prop selection until we know if the tach is accurate.
As I suggested earlier a prop with added cup may help the slip along with the 4 blade.

 

[dazk14]

Re: Proper Prop Selection

Is your prop shaft drilled and tapped on the end? If it is, then you can use a long hub prop with a 2 piece spinner.

The 16" diameter props is probably where you want to make a living with a lot of drop in pitch.

There are also props that have slots on the barrel, these likely ere a long barrel, long blade, but short hub as the slots allow for bending of the tab washer.

That said a 15" is still much better than a 14 which is for high speed applications. Have fitted a 4 blade 15x15 Volvo by Solas, with mixed results on a 20mph max speed boat. Not sure if I still have notes on it.

 

[dazk14]

Re: Proper Prop Selection

Here's a cheap place to start...used 15"x17"LH michigan short hub, long barrel for $38BIN +12.79ship.

It'll at least give you some data with a 15" diameter prop, so pulling the final number will be more accurate...or who knows, it might plane somewhat and solve your problem.

Easy to get your money back on resale. Of course 1st answer whether your prop shaft is end drilled.

Volvo Prop | eBay

 

[Scott Danforth]

Re: Proper Prop Selection

which engine? the 6.2/6.5 detroit, earlier 5.7 olds derived motor, or the newer duramax (isuzu) motor

if it is a non-turbo motor, they are kind of a dog with the 6.2 putting out 130hp, and the 6.5 putting out about 145hp.

The turbo motors were between 180 and 205hp in truck trim, however it is not a marine motor (different injector timing and compression)

There was a place in Switzerland that was marinizing the 6.5T

There is also Peninsular Diesel which marinizes them.

Banks is marinizing the Duramax