Any propeller experts here?

[captcrunch]

According to the factory my 1988 38c with 671 Detroit tibs came with 26x26” 3 blade props. They are long gone. When I bought the boat it had 26x24 4 bladers on it. I was told it should have 24” props. I don’t have enough diameter when loaded, but my yard is saying they think 26x26 3 bladers are to much wheel for the boat and are suggesting a 26x22.5 4 blader. Thoguhts?

 

[drburke]

Go to www.boatdiesel.com, get a membership, then use the calculators.

DAN

 

[Bob Bradley]

Randy Hale at Hale Propellers in Old Saybrook Ct would be a good source for info. 860-388-5051

 

[dsharp]

Why do they think the 26 x 24 is too much wheel? What rpm will the engines turn with the boat fully loaded? When you go from a three blade to a 4 blade you generally reduce the pitch by 1 inch which would put you at a 26 x 25. It’s pretty common to have to take an inch of pitch out after a boat ages a bit and equipment and junk gets added. Not sure why they want to take another 1 1/2 inches of pitch out unless the boat won’t hit the rated rpm.

 

[jim rosenthal]

I have had very good luck with General Propeller in Bradenton, FL- very accurate calculation and exc customer service for propr sizing.

 

[Captain Ralph]

So, What is wrong with the original 3 blade prop spec. If you are comparing original specs, should not your future questions be regarding a 3 blade wheel.
You have the diameter to help dock side and low-mid speed gripe on the water, but added surface friction is not going to buy you anything but out of factory specs.
I truly don't see any help in noise or vibration in this size.

Talk to a good prop shop. They make a living on propellers.
Boat yard folks just want to sell something (anything) IMO.

Just trying to help.

 

[captcrunch]

I’m being told the original 3 blade spec is to big for the boat to turn which seems strange

 

[Captain Ralph]

I’m being told the original 3 blade spec is to big for the boat to turn which seems strange

By a yard or real propeller shop??

You have to realize, the factory usually tries out many sets of wheels before they deliver a boat with a spec'd set. Usually has specs for mother-in-law loads also.

I wonder,, if technology has improved in this area any, would the factory have any new data on these requirements?

Just takes a phone call..

Anything could be better than Buba at the yard..

Please keep us up on what you find and how it works out..

 

[Jackman]

I would definitely ignore the yard and find a professional prop shop or even speak to the guys at Sams). If the answer is "too big" they really should not be your resource. There are so many factors to consider. Physical size....you want to avoid props so large that they come witnin a certain distance of the hull...mine are about 1" to 1.5" from the hull in the prop pockets..... or they might cavitate. You can always change pitch like others mention. I've found 4-bladers run smoother but might need tweaked back a bit on pitch to account for the added blade surface area... Dont forget cup...if props are spec'd closer to the max size you can handle remove some of the cup if it is suspected they are too large.

But forget my advice....I'm not a professional. What is telling them they are too big? If you cannot get max rated rpms for your engines at WOT then you need to tweak. If your egts are high (I assume you have turbos with that engine model) then tweak the props (remove cup or pitch until you're in egt range). If the yard guys are not mentioning egts, boost or or rated RPMs at WOT in their discussions definitely ignore them. Find someone who knows what they are talking about.

 

[Avenger]

Not an expert by any stretch, but your statements are confusing me. It appears you have 26x24 props currently, which I read as 26" diameter, 24" pitch.

Then you say you want 24" props because you don't have enough diameter?

Exactly how do the current props perform, and why are you looking at changing them?

 

[dsharp]

I’m with Avenger. Your question doesn’t make a lot of sense. There’s nothing magic about prop sizing. The computer programs will get you close but there are a number of variables that will throw the numbers off. Has a tower been added, extra fuel capacity, added hard top etc. Then you have to know the water line length and the weight of the boat, By too big I would assume that you mean too match pitch. You are going to have to run the boat at full throttle and see what rpm the engines will turn first then give that information to whoever is helping you with the prop sizing. I’m just a second gen buba so you may want to ignore what I’m telling you.

 

[cww]

Call a professional like Black Dog Propellers and buy the set of props from them. It isn't that much more expensive and then you'll know it's right. There are professional shops where for 50 years all they've done is props. That is who you want to talk to. Odds are they've done props for your model of boat many times. I wouldn't listen to some guy in a boatyard on this issue. I've been through this myself and it never worked out quite right until I called a prop shop and told them what was going on. In my case I couldn't reach rated rpm. It was bad engine alignment on one side combined with old cutlass bearings that were swollen making the shafts hard to turn. It wound up having nothing to do with the props. I put my original props back on and re-pitched the new ones back to factory spec so I have spares. I wouldn't have figured it out if the prop shop hadn't already done props for my exact model of boat before. After looking at the weight from the travelift scales and the rpm I was getting they told me something was wrong with the boat. And there was. This forum also helped a lot, I made a post about not being able to turn the shafts by hand and they told me it was causing my engine stalling issue, which it was.

My point is this: You are going down a money-wasting trail listening to guys in the yard and trying to figure it out yourself. You'll eventually get there doing it this way, but only after you go through multiple sets of props and haulouts that you have to pay for. Which...oddly...benefits the yard encouraging you to do it that way. It's cheaper to just let a professional get it right the first time. The normal techs in a yard do not have the required knowledge to do that, and neither do you. Or at least I didn't.

 

[captcrunch]

Sorry for being confusingBoat left Hatteras with 26x26 3 blade propswhen I bought it 7 years ago those were long gone and 26x24 4 blade props were on it.Was told wheels were to big, currently have 24x27 3 bladersNeed more diameter and was told to go back to a 26” diameter wheels.

 

[drburke]

Repectfully, Captcrunch when people say "you need more or less propeller" they are nearly *always* referring to the pitch, not diameter. (I am sure somebody will contradict me here

The first number is the diameter and second the pitch, thus it left the factory with 26" diameter wheels with pitch per turn of 26"

[For the record, the manufacturer sets the largest diameter that will fit without cavitation or resonance (usually at least 10% hull clearance) and then the pitch is changed as needed.]

Your boat shipped with 26" x 26" 3 blades. An appropriate 4 blade will be 1" or 2" less in pitch, or 26x25 or 26x24. Thus your 4 blade set was specified correctly.

When you were told they were "too big", that means you were overpropped in pitch NOT DIAMETER likely due to excess weight.

So you need 3 blade: 26x25 perhaps, or 4 blade: 26x23 possibly.

The yard was telling you to go down in pitch. As was said before, you really should start working with a prop shop to get the right tweaking.

DAN

 

[captcrunch]

Yes I understand they told me I had to much diameter. Now trying to get my hands on some larger wheels again.

 

[Captain Ralph]

Diameter is our friend.

 

[Bob Bradley]

I went thru this exercise many moons ago when I rebuilt the 327's in my old Chris. At the time, I was told that the distance from outer edge of the blade to hull may be no more than 15% of prop diameter. In that case, I had 17" wheels, so they could not be less than 2.55" from the hull. Obviously, we also need to be able to hit our top rpm with whatever wheels we use.