Inboard prop question

[rthomas]

I recently bought a '26 Tollycraft sedan, its heavy (9000+) not really a planing hull but they will if you want to burn the gas, the older ones like mine came with a conventional drive that left limited room for a prop of adequate size, I posted on the tolly forum about trying a 4 or 5 blade and didnt get much of an answer. Right now I think it has a 14x19p 3 blade and with this it runs about 14 kts at 3000 rpm it seems like I get a lot of slip starting about 2500, it seems to me that a more blades would help but I want to keep my economical cruise at 2200/8kts Anyone have experience with boats like this?

 

[rallyart]

Re: Inboard prop question

Talk to a company like OJ or Acme that makes CNC machined props. At 19 pitch you would be better with a four blade than a three if you want to avoid slip but a three blade will normally be more efficient. I don't have any direct experience with a boat like yours but those two companies do and make very good inboard props.

 

[jestor68]

Re: Inboard prop question

What sort of engine is in your boat? 3000 rpm at WOT doesn't sound like enough, even for a diesel.

 

[Lyle29464]

Re: Inboard prop question

I might be able to help a bit. What engine ( make HP) and gear do you have ?

 

[rthomas]

Re: Inboard prop question

It has a mercruiser 255 (350 chev) with a 2:1 reduction, it does get to about 4600 but I dont have a speed for that rpm, the newer models had the same power with a V drive, these had a steeper shaft angle that gave better clearance for a larger prop.

 

[steelespike]

Re: Inboard prop question

I think there is no doubt a prop with more blades should help your slip. Usually as you reduce rpm slip will go up
at 8 knots slip is around 53% but of course your using a lot less gas gph( gallons per hour). At 3000 slip is 40% Some of your slip is from trying to do planing speed with a hull not intended for it.
I can't really be specific but a for blade with some cup should improve the situation.The prop gurus may be along with some
specific suggestions. You might contact Michigan wheel. You might check your present clearance to see if you might squeeze a little more diameter.Probably your best speed may be 6 to 8 mph. The boat may respond to a prop with stern lift.

 

[Lyle29464]

Re: Inboard prop question

I would work with a few of the online prop calculators to increase the Diameter . This means you will be taking about the same off of the pitch if your engine is reaching top RPM. Every one always recomends a 10 to 15% blade clearence. I have had to go a lot less on some inboards and have never seen a problem. Remember you add 3/4 of an inch to the blades it is a 1 and 1/2 increase in Diameter. That boat really needs a 17 or larger prop. I would see if you can work with a prop shop. start with a 17X16 I am repowering an inboard now and I will have the prop shop cut of a bit of blade for clearance so I can pick up diameter and surface area. If you want really great control you will need the large prop. In the prop calculators try for a 33% slip. Do a search on "square prop" use it as a rough guide. First step is a reall good measurement from the hull to the center of the shaft at the correct angel. Post that and we can run some numbers. if you cant work with a 16 or 17 think 4 blades.

 

[rthomas]

Re: Inboard prop question

Diameter (clearance) is the problem with these early 26's, the Vdrive boats had room for a 18" prop, they are much more efficient in the 9-15kt range. I like the slow criuse but I know there will be times when that wont be practical.

 

[rthomas]

Re: Inboard prop question

I went through some of the POs paperwork on the boat and saw 14x12 as the prop that is on now, no mention of 3 or 4 blade. At the end of the month I'll have it out of the water and be able to know first hand.

 

[rallyart]

Re: Inboard prop question

If it's a 12" pitch a three blade should be fine. That would put your no slip speed at about 15 knots at 3000 RPM. At 6-8 you are still at displacement speeds which is why you get such good economy. Above that you are pushing a lot of water until you are on plane. Maybe your prop is okay.