More prop or lower gear ratio?
 

I just blew up my drive and I'm considering going from a 1.5 to 1.36. I currently run a Bravo 28p but I should be running at least a 30 or 32. At what point does increasing pitch start to become less efficient?

You're going the wrong way ( ratio wise) if you want to be able to run a larger prop with the same amount of power

My question is, should I get a bigger prop or go with a lower ratio? The 1.36 gears are stronger than the 1.5, but what combination gives me the best efficiency? Since the drive has to be rebuilt anyway now is a good time to ask this question. If I go with the 1.36, I can probably stay with the 28, If I go with the 1.5 I will need to step up to a 30 or 32 prop.

Are you under propped ?....if the 28 pitch prop is letting the engine run in the proper rpm range with a 1.5 ratio drive, and you change the drive ratio to 1.36 ( less gear reduction, faster prop shaft speed than the 1.5), you'll need to drop DOWN at least 1 size in pitch to maintain the same engine rpm.

If you are under propped and have about 1000 rpm left over, you could probably go to the 1.36 and use the 28 pitch prop.

Bravo's start to lose their efficiency above 32" pitch

It sounds like what he is saying is he is over his RPM range on 1.5's so he would have to prop up. Instead of propping up he wants to change the ratio to 1.36 instead. It should work, but I have no idea what the difference in RPM would be.

a 28p to a 30p is about 400rpms difference, then 30-32 is about another 400rpms, so you are saying that you could be possibly over spinning 600-800rpms? what are you spinning now and what is your ideal rpm?

It is generally more efficient to spin a large pitch prop slower than a smaller pitch prop faster. I'd keep the 1.5 and step up in pitch as required to match engine max HP rpm.

Intresting I ask this sincerely why do 1/4 mile boats use such small props? One more if I may can a prop go supersonic in water? Ahh why then as one goes up in HP do mfg's go down in ratio???? 375 hp with a 1.98 final?

Ask eddie knox if anybody can he has. (supersonic)

I mean the prop turning so fast it goes supersonic....

this is what is going to happen with your boat.. a 1.50 ratio will go from point A to poin B quicker..your boat will have more acceleration..If you switch to a 1.36 ratio your boat will be a lot slower going from point A to point B..

I have heard this from multiple sources, but always without any reason why. It's like putting bigger tires on your truck and changing the axle ratio to keep the true final drive the same. I guess what I'm really interested in is the science behind it all. Is there a point where the prop is spinning too fast and becomes less efficient? As was mentioned earlier, 1/4 mile boats run relatively low pitch props at very high rpm and they seem to have no problem accelerating. I'm trying to wrap my head around the hydrodynamics of a prop spinning through water and it seems to me that as pitch increases, efficiency would decrease as well. As you turn the blade to increase pitch you are creating a larger area for low pressure to form thus causing the water to vaporize. Once the water starts to vaporize you lose efficiency as you lose the suction effect on the back side of the prop. I would really like someone to explain the theory behind prop design in high performance applications. I am hitting my 6k rev limiter with the 28p prop, I have tried a 30p and the boat really liked it, I didn't have enough time and good weather to see what the max rpm was. I was at 5500 with 4 adults and a full tank and it was still climbing with the 30p so I'm thinking a labbed 32 would be perfect, or switch to the 1.36 and the 28p. I'd really like some real world experience regarding the different ratios, obviously keeping the same prop and changing ratios will reduce acceleration I could use some of that anyway. When I leave my harbor I gradually put down the throttle to reduce the strain on the drive. I start with the drive trimmed under and I have to hold the trim button up while I put down the throttle as the trim can't keep up with the boat. I usually hit 60mph before I stop trimming and I only bring my drive to just above level so it doesn't have far to go.

Having owned a Velocity years ago and knowing Steve Stepp, have had this conversation with him, if I remember right his thought was to turn the largest pitch you could to get the best speed, in your case here run the 1.50 gears and the higher pitch prop.

My understanding over the years was that you changed up the gears when you ran out of pitch. I had a 32 Velocity that I transplanted some bigger power into and ran out of pitch to work with, had to change the 1.50's to the 1.36's in order to stay off the limiters. Once the higher pitch props were available I went back to the 1.50's. Reality was that the top end was close to same either way, diff was that the acceleration went to the 1.50.s, overall it was the better set up. Truth was that I managed to trash the Bravos regardless of the ratio.

First and foremost, I stated generally....

Next a 1/4 mile boat, its drive set up and boat weight are significantly different as compared to most of the boats we discuss here, and specifically in this thread. 1/4 mile boats run significant hp in very light and small boats, with beams on the order of 4-6'. The most efficient prop would be a single blade prop, however this is completely impractical. Many drag boats run 2 blade props for efficiency and control. The downside to higher pitch props is the lateral and vertical lift components developed as rpm increases. For larger boats as mostly discussed in this forum that is not a major factor. In drag boats lateral and stern lift can often times lift the boat and "walk" it sideways to the point of no control. Watch the start of most drag boat races and watch the hull reaction out the hole and on decel. Its quite enlightening. In addition, because of the lighter weight a two blade prop can hold or carry the load for the HP applied. This would not be the case for the boat we are discussing here.

This question is a little nebulous as I think some people have, perhaps, a misconception about the speed of sound and how it relates to temperature and fluid media. In addition are you asking if the prop can translate through water at supersonic speeds or are you asking can the blade radial tip speed exceed supersonic speeds? Again would this be speed of sound in air or water and at what temperature? I don't want to belabor this point as it diverges from the OPs original question.

I haven't seen a specific case where HP goes up and drive ratio goes down UNLESS you run out of pitch for a particular blade set. I've never seen a 375 hp motor with a 1.98 drive unless you are talking about a Bravo III setup or perhaps a V-drive boat. This is again a completely different situation, but in the end it involves swinging (turning) a larger prop at a slower (rotational) speed, which generally is more efficient. The trade off here is superior dock handling, elimination of torque/paddle steering at the expense if higher gear case drag, weight and complexity.

Regardless, its all fascinating stuff to talk about. I wish the OP a quick resolution to his dilemma.

All of the single engine boats with hp motors that we tried the 1.5 and the 1.35 liked the 1.5 much better faster and better acceleration

Thank you, that's what I was looking for, real world experience or data of some type. I appreciate all of the input, it looks like I will stay with the 1.5 and move up to a bigger prop. This saves me a lot of wasted water time and money trying to re-invent the wheel.

And no one even has talked about gear ratios and moving your torque curve / rpms. Next topic ?

I agree with Bup. That is why I am going back down to 1:36 gears.

I'm listening...

I had this very issue in my 28 Nordic. I was trouble finding correct prop and the bigger I went (above 30P) the worse it handled, and I was limited by the RPM that I turned or could turn. I went with 1.36 and 28P. Boat goes faster, handles way better and I have room to grow. (although I've tried bigger props with the 1.36 and they don't work as well as the 28P.

Boat accelerates as fast as it ever did.

Good luck-

basically in a nut shell and not to go into huge depths here - as we all know gear ratios move your rpms up or down - your engine makes its peak torque levels down lower than your peak HP up higher (rpms) Torque is what gets these boats up and running. Moving rpm levels up or down moves your peak torque levels - you do not want your peak torque levels down too low or way up high on your rpm range. making peak torque high on the rpm range will make a pig of a boat for cruising - getting on plane, getting out of its own way and so forth.

To further look at it - for longevity of any marine engine, they will last longer running at 75% duty cycle for load periods of running. That duty cycle of 75% is within the range of your peak torque #s. I am basically talking about stock to medium performance set up marine engines built with production parts to above duty performance parts here.

I had the same hull as you and I ran a 1.36 ratio with a stock 30 pitch bravo 1 4 blade prop. The boat would run 92-94 mph @ 5200-5300 rpm all the time. If your looking for 6000 rpm with a 1.36 ratio you may have to go to a 26 pitch or lab your 28, depending on how much torque your making. The boat would get on plane and accelerate very good but it had a little help with a vortech supercharger making over 800 ft # of torque.

What he said^^^^^^ mostly:D

Going to a 1.36 from a 1.5 is around a 600rpm/3" of pitch difference.

I'm under propped now and hit the rev limiter with the 28p and testing with a 30p indicated that might not be enough either. If I get better acceleration with a 1.5 and a 32p then a 1.36 and a 18p then that makes the most sense for me.