TOPIC: prop pitch

i have twin bf25 they had 3 blade 9 3/4 pitch i went to a 4 blade 13 inch pitch not much power there had them adjusted to 11 inch that helped aleast get it on plane i want to go more will this help my top end i know you can,t take them back can anyone help

First of all, you'd be much better to post this is the main forum. This is the "Support" forum, for folks having trouble with the web site.

Now that I've re-directed you, here's some info folks would need to help at all.

What is the max rated rpm of your motors?

Do you have tachs for both motors?

How fast are you going at wide open throttle?

What are you gauging speed with (pitot tube, or gps)?

Good luck!

Frank

That's a great looking Bellboy Express Ralph, nice job bringing her back! Hope you share some pics of the project with us, and welcome to FG.

As Frank said, the folks will need some baseline info to help steer you in the right direction for prop selection. You made a pretty drastic change there, sounds like you may have gone a little too far up in pitch. Normally when changing from a three to four blade prop the diameter is decreased by at least a 1/2" or so to help compensate, that would be something else to consider. You're right about not being able to return props (normally) once they've been run, but a good prop shop may be able to fine tune what you have to maximize the performance.

You didn't tell us, but what was the reason for the change to start with?

That's a very nice looking BellBoy. Unfortunately, 50 HP isn't going to give blazing performance. The fact that you have twin engines totaling only 50 hp makes things even more limiting. If you had a single 50 hp engine on the boat, it likely would be faster. Twin motors weigh more, and there is twice the lower unit drag. If set up correctly, I'd guess that your boat might reach speeds from 25 to 30MPH top speed. To get off idle power & thrust, you need a low pitch propeller. To go fast, you need a high pitch propeller. Unless you have tons of reserve power, you don't get both. You can't have your cake, & eat it, too. Your best bet is to make a compromise between pulling power (torque) and speed.

thanks i really have learned this buy the seat of my pants i thought if one is good two would be better and if three blades are good four should go faster my top speed was 12 mph after changeing the pitch i got it up to 20mph i really do appriciate your time

thanks mark your info was helpfull here are somemore pics drop us a line sometime

As noted by PC1000 more info is needed, rated RPMs for engine and what you are achieving being the most important. 4 blades are generally for pontoon boats and ski boats that are pulling or pushing a load. They do have more drag that will affect top end. Many speed boats run two blades for that reason. I would go back to the original three blade and get us the asked for info. Some times it is better to ask questions before the purchase than after.ralph wrote:

thanks i really have learned this buy the seat of my pants i thought if one is good two would be better and if three blades are good four should go faster my top speed was 12 mph after changeing the pitch i got it up to 20mph i really do appriciate your time

Another thought after studying the picture of your beautiful cruiser there Ralph: The motors appear in the pic to be riding a little low. (I can see the top mounting bolts have multiple holes - are adjustable, can't see the bottom ones.) The cavitation plate just above the prop should be even or slightly (1" or so) above the hull (just forward of where the motor is.) Raising them a little (looks like you could go 2" easily?) would reduce the drag created, increasing your speed some. I realize that's a big boat, and you're not going for blazing speeds here, but it could make a significant difference. Just a thought....

If you came up with a couple of these that would fit your motors, you could adjust the pitch and optimize them, then get the right props. I need to do that next year on the Aqua Ray

wow....never seen one of those. who made them and when?

Memory is a little foggy about them, but I believe they were an early aftermarket attempt at variable pitch props. Didn't last long, mainly because of price and durability. Land and Sea made a self-adjusting pitch-switching prop in the 80's that was reasonably priced, worked great, but was badmouthed by idiots who didn't want to take the time or put in the effort to tune them properly, or abused them by over-revving them (Clearly stated they were NOT intended to turn over 3000 PROPSHAFT rpm's) When set up properly, they were positively WICKED! Knew a guy with a mid-80's 19' Four Winns with a 175 Johnson that had one. He said he had it set for 22 pitch upper and 11 pitch(!!!) lower. Ever see a 19' bowrider almost prop-walk out of the hole?

This was the prop that was on the Aqua Ray when I go it. I was told it was for figuring out what pitch is best for a specific boat and motor combination. There is someone on the site that has an NOS one.

Keep that prop!!!!! Very helpful to many people to prop their rigs. Not recommended to run all the time, just for testing.

The Land and Sea prop was one great prop. I put one on my 84 vagabond up in toronto back in the 80s. I have a 125 hp. merc on it and the performance was outstanding. You really had to hang on when you gave it everything on a holeshot. It moved! The other test prop is also great from what I have heard about it. I saw 2 of them in a small shop in western Iowa last summer and I think I will take a drive next week and see if they are still there. They are really great at saving alot of time and money on props. Skip.