Same RPM, same speed with 17 and 22 inch props.

[Ram_Boat]

I decided to explore with raising the engine. To my surprise 8 inch above the transom worked still not too high.
The question remain; why do I get the same speed and same rpm with different props. There must be some one with knowlwdge to provide me a clear answer?

[dompie99]

The fact of the matter is that an Aluminium prop is NOT MEANT TO BE ABOVE THE WATER without loosing all it's grip. That's the way it's always been or almost.
Stainless Steel prop with the ',tube'' around the center hub will help keep the exhaust gazes away from the blades. (so to speak ).

There is a multitude of props style, type and design ranging in all prices. Hoss are excellent, Cleaver, Choppers, thru hub or not, big ears choppers, or small ears, fat shaft or small shaft depending of your engine and your goal. You will be able to find more threads on this board about the behaving of these types of prop.

If you want to go '' faster'' with your rig, as we discussed earlier, jack up the engine and increase the pitch untill your water pressure gauge shows you that it starts to loose pressure, than go back down a little. Obviously, a nose cone would be to consider because the water intake, usually located on each side of the lower unit will have a tendancy to ''run out of water '' when raised, that is the reason or the main goal of this water pressure gauge in your set-up.

Some people here, block the two or three top holes and drill one or two lower than the bottom ones which I did not try, I was more on the ''nose cone '' side.

They will tell you that is becomes a ''balance'' between the height and the prop pitch to keep it in the HP range of your engine if you remain in the 50:1 oil to gaz ratio and if you want to keep your HP maker ''together'' longer. You will also notice that the ''hole shot'' is more and more ''awfull'' as the engine is raised on the transom but you can't win them all as they say OR, in french, you can't have the butter, the money of the butter and the but of the milkman's wife.

Hope this sheds some light on your '' goal ''. That was my .02$

[Ram_Boat]

Thanks Dompie99 for the reply. There is plenty for me to search with your answer but there is one part I could not find an answer.

By raising the engine to get the prop out of the water I am reducing the amout of blades in the water to unload it, that result in higher rpm with all the benefit from this. Would it be easier to keep the prop in the water and use a prop with less blades? Less blade could be a smaller diameter or a 2 blades vs a 3 blades prop. I would not need to worry about loosing water pressure and save stress on the transom.

my 0.02$ question

[Euroski]

Moving engine up reduces friction of gear case also... and you go faster.

[Ram_Boat]

Yes there will be reduction of friction with less gear case in the water but if we are talking about the portion below the cavitation plate the gear case is pretty slim. There will be gain in RPM because the blade area in the air have no restriction thus unloading some power. On the other hand, asking a prop made to run in water to work partially out of water will reduce the efficiency. We are talking with no numbers making this really a guessing game. It is costly in terms of time, money on props and increasing stress on transom.
How much more resistance if the gear case is 5 inches lower?
How much trust do I get from a prop surfacing 2 inches above water?
How much trust do I get from a prop fully submerge same pitch but 10 vs 11 inches diameter?
Is there a way to avoid the guessing game?

[dompie99]

Yes there will be reduction of friction with less gear case in the water but if we are talking about the portion below the cavitation plate the gear case is pretty slim. There will be gain in RPM because the blade area in the air have no restriction thus unloading some power. On the other hand, asking a prop made to run in water to work partially out of water will reduce the efficiency. We are talking with no numbers making this really a guessing game. It is costly in terms of time, money on props and increasing stress on transom.
How much more resistance if the gear case is 5 inches lower?
How much trust do I get from a prop surfacing 2 inches above water?
How much trust do I get from a prop fully submerge same pitch but 10 vs 11 inches diameter?
Is there a way to avoid the guessing game?

Some of your statements are absolutely true.

A surfacing prop is meant to do just that, surfacing, it uses the lost of ',traction'' to spin up the engine of which you have to compensate with more pitch on the prop. Trial and error is, sometimes ' the answer to this dilemna. Unless, of course, someone has been into this path and ''been there done that ''

[sschefer]

Here's something I just found on one of my pleasure boats. It's running a Yamaha F150XA behind a 20' Kingfisher Discovery 2025. I had a Black Max 14.5x21 on it and was hitting 49.4 at 5800. I put a 19 on it to see if I could hit the 6000 mark. I hit 49.4 at 5800 again. That's pretty much impossible and had me scratching my head for darn near a month. It wasn't until I ordered a Yamaha Reliance SDS prop and noticed that the hub to gear case clearance was much tighter. I did a little research about the problem and found that the exhaust was blowing out between the prop and the gear case and causing excessive slip.

What it all boiled down to was the wrong thickness of thrust washer. A simple 28.00 part. You might just check that and see if maybe your problem could be the same as mine was.

[Slimm]

Yes there will be reduction of friction with less gear case in the water but if we are talking about the portion below the cavitation plate the gear case is pretty slim. There will be gain in RPM because the blade area in the air have no restriction thus unloading some power. On the other hand, asking a prop made to run in water to work partially out of water will reduce the efficiency. We are talking with no numbers making this really a guessing game. It is costly in terms of time, money on props and increasing stress on transom.
How much more resistance if the gear case is 5 inches lower?
How much trust do I get from a prop surfacing 2 inches above water?
How much trust do I get from a prop fully submerge same pitch but 10 vs 11 inches diameter?
Is there a way to avoid the guessing game?

Some props are designed to work with the top blade(s) out of the water. This allows them to . believe it or not, grip better and work more efficiently. My setup gained MPH and actually lost RPM when I trim it up and up and UP. I thought I was going to surface the prop and blow it out but actually when I surfaced it, it began to work as the manufacturer intended and it bites better and loaded the engine while gaining MPH. A thru-hub prop needs to be deeper in the water than an over-hub prop.

in regards to your question about two different size props running the same RPM and MPH. There are lots of factors that could allow this to happen. Maybe the higher pitch prop needs some work done to it in order to make it bite the water better. Maybe the lower pitch prop is of a different manufacturer and is designed to have more bite than the higher pitch. There are many factors that come into play with a situation like that. If you are seeking the best performance, then you should do what has been recommended already and ditch the aluminum props and start with stainless steel which is stiffer and less likely to flex or distort under a load.


Just my 2¢

Slimm