Prop help Volvo Penta 5.0GL with SX Outdrive

[RXWingman]

Hi All,

Been going down the black hole of prop advise and unfortunately I've not found my answer. I have a 2001 Reinell 203 with Volvo Penta 5.0GL and SX Outdrive. I bought the boat used and it came with the 15x17 black aluminum prop.

Recently I've noticed a little more vibration at cruise and grinding noise from the outdrive. The prop has some noticeable nicks and dings, not huge, but appropriate with wear for a 18 year old boat.

Engine has probably 350 hours on it, runs great and I've had no major work done other than regular service of impeller, oil change, spark plugs, etc. The oil in the outdrive was performed a couple years ago, and the boat hasn't seen much usage. Outdrive fluid still looks brand new, and is at the proper level.

Assuming my outdrive is not going out, and the vibration might be related to the prop, I've been trying to determine the proper size and pitch for my engine and boat. Currently on smooth water, and approx. 5,500 rpm I can hit 45mph, but it seems like the engine is reving too high and at that speed. Boat is 20.5" long, weighs 2700 lbs, usually has 20 gals of fuel, minimal gear and 2-3 passengers.

I've currently moved the boat to salt water (I know, not optimal) and wondering if a stainless prop, either 3-blade or 4-blade would be better for my situation, and also wondering how I determine what size and pitch prop I should have on my boat for maximum performance, and smoother cruising? Hole shot is not that important, however top speed, and high cruise speeds would be preferred.

It use to be a ski boat, so holeshot was important. I have no idea if the 15x17 prop was the prop it came with, or if a "ski" prop was added by previous owners. Now, we just mostly use it for cruising around the SF Bay area, and the occasional some pulling of tubers at low speeds.

Any help would be appreciated.

 

[jimmbo]

You are over revving by quite a bit, WOT range for that engine is 4400 - 4800. you probably need about a 21 inch pitch for that combo.

 

[RXWingman]

You are over revving by quite a bit, WOT range for that engine is 4400 - 4800. you probably need about a 21 inch pitch for that combo.

Thank you Jim. Because of the salt water, and vibration issue, should I go with a 4-blade stainless, and will the 21 pitch help or hurt the cruising speed and overall performance. I understand that the larger pitch will reduce RPM, but should I expect better performance with a higher pitch prop?

 

[jimmbo]

Adding pitch will reduce the WOT rpm, and increase the mph for a given rpm. It will also reduce how fast the boat accelerates. How it affects top speed is somewhat more complex. Even though the speed per rpm does increase, so does the load on the engine so the rpms drop, Often increasing or decreasing Pitch results in the very little difference in top speed, just changing the RPM it is attained at(when the different props are still within the engine WOT range) In your case being that you are quite a bit above the WOT range, your engine is actually producing less HP than it would at 4800 rpm, reducing the speed to 4700 - 4800 the engine will be making more HP and be able to swing a higher pitch prop more efficiently and you should see a few more mph. One more Caveat, that too is assuming you are using the same lineage(model/brand) of props in all cases. Switching brands and/or model of props can and often does make it into an Apples to Oranges Comparision.

Higher performance means different things to different people. To me it is Rocketsled Acceleration and High Top Speed, Others feel differently. There is a prop that does well for what I want, Others on the board also praise it, too. A couple of people have said they don't like... It is not an inexpensive prop but to me it is still VFM.

Adding an extra blade will reduce top speed. A 21 4 blade will have lower rpm, thus lower top speed. Myself I avoid 4 blade props like the Plague, I just don't like giving up top speed for any thing a 4 blade may offer, and to me they offer nothing. Plus they look obnoxious, 2 blade, 3 blade, even 5 blade props look great, but 4 just looks out of place

 

[Scott06]

Adding pitch will reduce the WOT rpm, and increase the mph for a given rpm. It will also reduce how fast the boat accelerates. How it affects top speed is somewhat more complex. Even though the speed per rpm does increase, so does the load on the engine so the rpms drop, Often increasing or decreasing Pitch results in the very little difference in top speed, just changing the RPM it is attained at(when the different props are still within the engine WOT range) In your case being that you are quite a bit above the WOT range, your engine is actually producing less HP than it would at 4800 rpm, reducing the speed to 4700 - 4800 the engine will be making more HP and be able to swing a higher pitch prop more efficiently and you should see a few more mph. One more Caveat, that too is assuming you are using the same lineage(model/brand) of props in all cases. Switching brands and/or model of props can and often does make it into an Apples to Oranges Comparision.

Higher performance means different things to different people. To me it is Rocketsled Acceleration and High Top Speed, Others feel differently. There is a prop that does well for what I want, Others on the board also praise it, too. A couple of people have said they don't like... It is not an inexpensive prop but to me it is still VFM.

Adding an extra blade will reduce top speed. A 21 4 blade will have lower rpm, thus lower top speed. Myself I avoid 4 blade props like the Plague, I just don't like giving up top speed for any thing a 4 blade may offer, and to me they offer nothing. Plus they look obnoxious, 2 blade, 3 blade, even 5 blade props look great, but 4 just looks out of place

Jimbo nailed it right on with all the variables and trade offs you're balancing in prop selection. Key point is how you use your boat is very individual. Best approach is to find a prop shop who will let you try a couple of configurations and pitches. your boat, power plant, load, and how you use it will determine what you buy.

 

[QBhoy]

Hi there. Before I even got to your rpm in the post, I pretty quickly assumed you were way way underpropped. Your engine is pretty seriously over revving. I’m surprised it even allows it. Even on a GL carb. You need a 21” for starters and go from there. If not, certainly at least a 19”.

 

[jimmbo]

The carb VP puts on the 5 liter is the Holley 500 cfm(350 cfm when flow rated like a 4bbl) 2 bbl. It is pretty much sized perfectly for a 5 liter turning up to 5000 rpm. Of course the carb can and will flow more than the rated cfm, but with a higher manifold Vacuum. Add to it that the engine's Volumetric Efficiency is dropping once the Engine exceeds the Peak Torque rpm, it might not even be flowing the rated CFM any longer when highly over revving