3 blade prop question

[Bill]

Heard from other 28 owners that a three bladed prop is an asset
when stopping and turning are crucial.
I repowered to a yanmar two years ago with new prop and new
shaft. Prop is two bladed.
Would a 3 bladed prop be advantageous in any way?
Thanks! Bill

[Tom in Cambria]

Everything is a trade off. A three blade prop will give you more thrust and more stop in reverse, but it also gives you more drag when sailing and not running the motor. A two blade prop "hides" behind the keel when sailing w/o the engine on. With a three blade, one of the blades is always sticking out from behind the keel. Depends on the kind of sailing you do. I have a 3 blade on my 31, because to me the extra push when motoring is worth the drag when sailing.

[Oswego John]

Hi Bill,

I wonder if you realize the depth of your question. Your friends are correct in what they told you. Sure they grab more water than a two bladed prop but just what are you gearing your boat up for, fast stopping and turning or hull speed and economy without undue taxing and strain on the engine. But they didn't go all the way and mention the ramifications involved in stepping up to a three bladed prop.

The very best advice I can give you is to have a true expert, not a self ordained expert, give your boat a thorough checkup.

Are you pretty much satisfied with your boat's auxiliary performance as it is? The prop should be matched to the engine, the tranny reduction gear ratio, the prop, the prop diameter, the number of blades, the blades width and the blade's cups.

Before anyone messes with your prop, it might be wise to make note of the engine mfgrs RPM with a wide open throttle (WOT) in neutral, under load in forward and also in reverse. Make note of the boats speed at various RPMs so as to have a basis of comparison with other prop experimentation.

Changing props can lead into an expensive trial, even if someone does it while the boat is in the water. You say that you swapped engines and props two years ago. Something tells me that some thought was given to what prop was selected to match the new engine. If you are happy with the status quo, don't fix something that isn't broken. Just go sailing.

Good luck,
O J

[Oswego John]

Tom,

It's good to hear from you and others that haven't posted in a while. Winthrop is back again with his sage advice. We heard from Andy D. this past week. Reports are coming in again from sunny Annapolis. I have to admit that I have a concern when we don't hear from the regulars for a while. I hope all is well with them.

So youse guys we're waiting to hear from yas, so hit the keyboard or I might have to give uncle Vito a call. Ya know waddimean?

O J

[bill2]

Arhggg . . .


Well being of the SO expert with no credentials nor training but being known to overhear conversations in low tones in taverns of questionable patronage I'll offer up a few hushed words uttered by a evil sort with a peg leg in a black tricorner hat and eye patch thusly :

Well m' laddy the salt that'll go to sea with two blades 'is looking for speed to out run the crown's man-o-war even ta lining the blades up with the keel but the bloke that'll have 3 blades be looking to motor mor' en sailing 'cause he'll always be draggin 'em along and the foldin' 3's can't but hardly back outa their own slip for all the rum in the islands - but laddy steer clear of the pale one th' has no blades 'cause he'll sail round the world on a half days rations and make more telling the tale of it then you'll ever fill your chest with . . .

Now that's my story and I'm sticking with it . . .


( FWIW - and no offense intended to anyone as most seem to use what came with their boat )

[Dick Barthel]

A two blade prop "hides" behind the keel when sailing w/o the engine on.

I think this is only true if you go to the trouble of marking your shaft and rotating it to "hide" the prop which you might do for racing but probably not for cruising. If you don't go to that trouble you could have both blades exposed depending how the prop stops. My point is that the extra drag caused by a three blade is probably minimal in the everyday reality of cruising.

When I repowered I went from a two-blade to a three blade on the recommendation of the installer who consulted with the engine manufacturer and Michigan Wheel. The performance under power is fantastic but I'm not sure how much of that can be attributed to the new engine (13.5 hp vs. 6.5, two cylinder vs one) vs the new prop. I haven't noticed any difference in sailing speeds.

Dick

[Dick Barthel]

The prop should be matched to the engine, the tranny reduction gear ratio, the prop, the prop diameter, the number of blades, the blades width and the blade's cups.

I think John's right on the money in that it's complicated and requires adjusting for quite a few variables. When I did my research and came across some formulas to apply, I discovered that the gear ratio has a tremendous effect on things. I obtained a copy of the Propeller Handbook by David Gerr (quoted by Vigor and Calder) and learned alot but in the end I listened to the experts and it turns out they were right. I would suggest some reading though just to make sure the people you're dealing with are really experts.

We had some good threads on this subject last fall if you search on "repower and 25D".

Our yard repowered my boat and a CD 27 both with Beta Marine 13.5s. I ended up with a three blade prop and the 27 a two blade I was told primarily because the 25D is one ton lighter.

Dick