Wakeboarding Prop selection - My new 4 blade doesn't seem to be turning enough water

[NooBoarder]

In short, I just purchased this boat that came with a mirage plus stainless 3 blade 19P prop that I believe is 15.25" diameter. The boat runs great with trim tabs but for wakeboarding I pull them up to get a nice wake and the boat struggles to get on/stay on plane. I'm not concerned with top end at all so I bought a 4 blade prop of the same pitch 19P 14.25" diameter. Now if I'm not carefull I can get the prop to 'slip' and the rpms will shoot up without any acceleration and the hole shot is only marginally better at best. Is the diameter change that big of a deal? I couldn't really find a good 15" diameter 4 blade 19P prop.

Boat
2001 Starcraft Nexstar Mach II 1910 GT

Engine
Mecruiser 350 MAG MPI - 300HP -5200 Max RPM's WOT
Boat would do about 60 mph at 5000 rpms ( I think it was) with stock prop - 3 blade 19P 15.25" Mirage Plus S.S.

OutDrive
Bravo One


In summary, I don't think my prop is turning enough water for the size/weight of my boat even though I kept the pitch the same. Do I need more pitch, bigger diameter, or is there some other detail of design I'm overlooking.

Thanks in advance for your help!

 

[Slow Ride]

Re: Wakeboarding Prop selection - My new 4 blade doesn't seem to be turning enough wa

My experience is that diameter plays a huge role! I'm not a ski or wakeboard guy but I have a similar experience. Let me explain. I had a 24ft champion bay boat that I occasionally took 30-40 miles into the Gulf of Mexico. I noticed that the boat would fall off plane easily when it was loaded down. Now, I was not going very fast. The rough waters held me to 20-25 mph on the way out on most days. Inevitably a large swell would come along and when the boat was climbing up the swell (much like the load of a 200lb buddy being dragged behind you) the boat would settle down and fall off plane. If I gave her more throttle...... wham, prop would break loose and RPMs through the roof! Sound familiar? The answer to my problem ended up being a prop that was a mere one inch larger in diameter. It was like night and day! I wont say I never had the problem again, but it was so rare that I never gave it any thought and it only happened on the tallest and steepest of swells from that point on.

 

[MaPaHa]

Re: Wakeboarding Prop selection - My new 4 blade doesn't seem to be turning enough wa

Does the prop have exhaust ports on it? I'm not sure if it is ventilating, losing its bite or just doing what a prop with exhaust ports does naturally. If you haven’t talked to Power Tech I would give them a call because they have a big selection of 4 blade props. I run one of their large diameter “big ear” off-shore series on my tri-toon and it holds good everywhere even in the turns.

 

[Texasmark]

Re: Wakeboarding Prop selection - My new 4 blade doesn't seem to be turning enough wa

Folks on here that talk about 3 vs 4 blade props usually drop the pitch one inch when going to the 4 blade prop from the 3 blade. This is not including a decrease in diameter which partially defeats the extra blade effect.

Falling off plane under load, or having a tricky "just on plane" attitude....cut back it drops off....gas it enough to get back on and it runs away requiring you to cut back, etc., etc.. is definitely a pain in the watoozie.

Large diameter, shallow pitched props are what's used for pushing heavy loads, and having your boat just under plane to make a big wake is certainly the worst possible spot for loading the engine. So I think you made the wrong choice. I think you would have been much better served by maintaining the same diameter and reducing the pitch to offset the extra blade and while you were at it a little more to improve your performance....hold the same old diameter, drop to a 17P 4 blade.

You changed 2 variables...# of blades and diameter and weren't happy with your previous performance. So, you more or less wound up back where you were when you started; unhappy with your performance. That is why my recommendation is as it is.

I am currently running a 21p on my boat but I have a 24 that I ran before. I can get my boat speed back up to where it was with the 24 but my rpms go from a tolerable 5700 to an intolerable 6500. I know how fast the engine will push the boat with the 24. I know that I can run that speed if I choose to use that prop! If I drop off 10 mph from top end (50) I can't tell it by looking at anything but gauges. So my point is, get your prop right for you water toys and enjoy since that is your reason for the thread. If you forget what your boat can do, put the original prop on, remind yourself, and take it back off. Make sense?

If your engine doesn't have room at the top to run at more rpms that currently, just back her off to where you can run at the upper limit of the recommended rpm range....like I said. If you don't look at the gauges, you'll probably never know the difference in mph.

HTH,
Mark

 

[NooBoarder]

Re: Wakeboarding Prop selection - My new 4 blade doesn't seem to be turning enough wa

Thanks for your help guys. I think part of my issue is that this prop doesn't like to trim as high as the last prop, I was able to force it to 'slip' by maxing out the trim up. I'll have to look and see if I can change the trim limit to match the new prop.

Either way, I'd still like to be turning more water as I didn't get the improvement I was looking for. I'll check with PowerTech also. Not sure if the prop has exhaust ports or not, my first thought is no, but I have the corsa 4" cutout option so for the most part the exhaust will exit the ports on the sides.

Thanks
Justin

EDIT: Thanks Mark, I didn't see your post when I had written this reply. It makes sense that the reducing the diameter could basically cancel out the 4th blade, that's why I chose to maintain the 19P instead of dropping pitch to 18P. I will say the 4 blade definitely improved my hole shot and the ability to stay on plane.

With all these variables you would think someone would try to simplify the selection with a volumetric flow rate per rpm. I calculate the 1" pitch diameter change to be about a 15% reduction in surface area, however the 'outer' surface area is what's spinning fastest with respect to the center so its still a bit of a guessing game.

 

[dazk14]

Re: Wakeboarding Prop selection - My new 4 blade doesn't seem to be turning enough wa

Which specific 4 blade prop did you buy?

 

[NooBoarder]

Re: Wakeboarding Prop selection - My new 4 blade doesn't seem to be turning enough wa

Which specific 4 blade prop did you buy?

Aluminum - Solas Amita 4 - 4 Blade 19P 14.25" Diameter Part Number 73854

 

[Levinz11]

Re: Wakeboarding Prop selection - My new 4 blade doesn't seem to be turning enough wa

I went from 3 blade 21 to 4 blade 19 Solas. Waaay better holeshot and low speed planing. I do, however, keep my drive trimmed all the way in to lift the stern. Why are you trimming out when towing?

 

[Tail_Gunner]

Re: Wakeboarding Prop selection - My new 4 blade doesn't seem to be turning enough wa

Well if you had a mirage ss which does a large diameter and is a agressive prop going to a aluminun small diameter prop will be disappointing. The mirage's are known for how well they hook up a good switch would have been a revolution 4 again a large diameter prop. The trouble your experincing is you started with one of the best SS props out there to improve that a rev 4 would have more hookup.

 

[NooBoarder]

Re: Wakeboarding Prop selection - My new 4 blade doesn't seem to be turning enough wa

I went from 3 blade 21 to 4 blade 19 Solas. Waaay better holeshot and low speed planing. I do, however, keep my drive trimmed all the way in to lift the stern. Why are you trimming out when towing?

Sorry I wasn't specific enough, I keep trim all the way down while towing and definitely have an improved holeshot & plane sustainability, but I wanted a more aggressive change. I'm only trimming up/out at top speed which honestly isn't something I'll be doing often.

Well if you had a mirage ss which does a large diameter and is a agressive prop going to a aluminun small diameter prop will be disappointing. The mirage's are known for how well they hook up a good switch would have been a revolution 4 again a large diameter prop. The trouble your experincing is you started with one of the best SS props out there to improve that a rev 4 would have more hookup.

That makes sense, the mirage is defintely a great prop, my last boat had one and it could do anything, only 135hp and I could pull 7 passengers and a wakeboarder like it was nothing!

I'll check on the rev 4, right now I'm really struggling to find a 4 blade 15" dia. in the 17-19P range.

Thanks again to everyone for the info.

 

[Bounce19712]

Re: Wakeboarding Prop selection - My new 4 blade doesn't seem to be turning enough wa

Sorry I wasn't specific enough, I keep trim all the way down while towing and definitely have an improved holeshot & plane sustainability, but I wanted a more aggressive change. I'm only trimming up/out at top speed which honestly isn't something I'll be doing often.



That makes sense, the mirage is defintely a great prop, my last boat had one and it could do anything, only 135hp and I could pull 7 passengers and a wakeboarder like it was nothing!

I'll check on the rev 4, right now I'm really struggling to find a 4 blade 15" dia. in the 17-19P range.

Thanks again to everyone for the info.

I bought a SOlas 4 blade 15Px16" for a designated ski prop for my 26foot/mercruiser trs outdrive. It is for forty yearold fatguys (like me) to avoid long duration torpedo hole shots.

try SOlas or Michigan wheels...

 

[Texasmark]

Re: Wakeboarding Prop selection - My new 4 blade doesn't seem to be turning enough wa

. I calculate the 1" pitch diameter change to be about a 15% reduction in surface area, however the 'outer' surface area is what's spinning fastest with respect to the center so its still a bit of a guessing game.

Don't bet your shorts on it. I have a 21P Hustler SS and a 24P XP Hustler SS and the prop surface area of the 24 XP is quite a bit more than the 21. If the 21 had proportional surface area to the 24 I wouldn't have had to buy the 24 to get my rpms down to a reasonable number.

Mark

 

[Slow Ride]

Re: Wakeboarding Prop selection - My new 4 blade doesn't seem to be turning enough wa

I've read through all of this and have an opinion (no kidding!) and a visual. In an earlier post, I mentioned my situation and how it was resolved. You said you wanted to "turn more water." The diameter is a huge issue! Some of our friends above stated this in several different ways. Adding an additional blade (or two) will give you a better hole shot. By the way, I don't think you need a 5 bladed prop. Just making a point. It's simple, there are more "paddles" in the water and if you over trim the boat, you will have an additional blade in contact with the water. That said, the diameter still plays a bigger role for your situation. Imagine this..... A one inch water hose with water flowing out at a constant speed.... lets say 10mph will have a certain resistance or thrust. A fire hose with a 4 inch diameter flowing at the same "speed" will obviously have more thrust. Even though the speed of the water is the same, the recoil on the hose is greater on the larger hose. You will have to reduce the pitch due to additional drag and weight, that is a given but the total diameter of the thrusted water is greater and therefore will increase your ability to haul heavier loads and maintain a constant speed. You will run at slightly higher PRMs but not anything noticeable. We do it all the time with our larger offshore boats. Guys (fishing buddies of mine) complain that the boat struggles in heavy seas and cannot maintain speed, or falls off plane, or the driver has to increase throttle the decrease then increase........ The answer is almost always larger diameter props with slightly less pitch and often times the addition of a blade from three to four. On my fountain, I lost about 5mph on the top end but gained plenty in the way of stability when going offshore and loaded with 200 gallons of gas and all the tackle, food, drinks, and stuff 5 guys could possibly use on a 12-15 hour fishing trip.

I think the advice above which says your should have stayed with the same diameter (or slightly larger in my opinion) but with less pitch is good advice.

Dave