using larger props to lower slip #'s
 

Boat is a 98 42 fountain, #4 drives 1.5" above the bottom. Boat planes great, never blows out the props. Slip is alittle high with 17" props. Have another set to try that are 17" but ran out of weather. I know i can space the drive down but would rather try to get it to hook up better with props. These are all 5 blades so far. What negative effect would 17.5" props have?

Are you on the limiters with that much slip? From what I've experienced a higher pitch will drop slip a little, maybe a couple percent, maybe. If you don't want to space down try bigger diameter or more blades. Both will take more power to spin, lowering rpm, but with that much slip I would think you'd have some rpm to lose.

Yea that's what I was getting at larger diameter. 17" vs 17.5" diameter. I was curious if it will cause any ill effects on the handling of the boat, too much stern lift Etc. In theory in my mind larger props could possibly equal lower speeds due to surface area and drag. Same reason that all things equal a 4 blade is faster than a 5 blade because of drag.

One of the prop shops around here will be able to give you a solid answer but being 1.5" above and that much slip I wouldn't think .5" bigger would hurt.

It becomes a trade off and search for the right balance of slip and speed. I think if you give up a little pitch, increase diameter to lower slip you will be ahead a few mph. Not giant gains like we'd like to see but a bit. You won't be able to spin as big of pitch, but slip will be down (good for the drives) and more efficient. A while back I tried 22-28 pitch 4 blades on my hawk, bravo and hydromotive, and 1" spacers. The best combo was no spacer and 24 bravos, the 26's pulled the same speed but took forever to get there. As did the 1" spacer. Slip only went down a little bit (1-3% I forget ) and took a long ways of hanging it out to get the last few mph. The race boat, 23 warlock has 3% slip at wot, but it is significantly slower than other warlocks with similar power supposedly are. Could a little more slip allow a bigger pitch and some more speed???

Thanks for the input Jd. I'm bummed out I didn't get to finish prop testing before it got cold. Spring can't get here soon enough!!

Indysupra,

I don't mean to hyjack your thread, I also have a question on diameter, which applies to your post, I'm just looking at going down in diameter. Ive tried a bravo1 26p and 28p on my setup. The 28 runs the best as far as cruise speed and WOT gps numbers and the slip is very good. My problem is that I'm running 200-300rpm low at WOT. Could I have the prop blades trimmed slightly to try and pick up the extra rpm to try and pick up a touch more speed?

Lab Finishing with the correct recipe can increase rpm by 100-400 rpm and increase speed depending on the recipe used. The recipe determination depends on current performance numbers.
If you would like we can discuss the options in detail on the phone. We do not charge for prop and set up discussions. We can more than likely find you a alternative you desire.

Brett

The easy answer to this question is you will get more stern lift. The "X" dimension on your set up is high. There are options for you to debate. Your boat was more designed for 4 blade props. We were just producing 5 blades at Merc Racing when your boat was built.
Knowing more performance information on your set up will help to determine the best route.
Call me anytime and we can discuss the options.

Brett

On my boat (even with bottom) 17.75x 27 4 blades = 60mph at 4000rpm.

Tried a 17x30 4 blade . took 5 minutes to get on plane and to my surprise no difference still 60mph@4000rpm but slip way up.(That prop had way less cup also ) My boat likes a lot of diameter and cup.

If you called Herring (about a straight bottom cig) they`ll tell you need at least 18" diameter... more blade in the water

Getting prop info here unless you are talking to people that have similar boats to yours is going to be worthless.
Brett at BBlades is going to be your best advice or someone that races BIG boats WITH surface drives.

Prop slip on a big heavy boat is better for the drives than low numbers. More so on boats that have the drives buried.

Bigger diameter and littler pitch usually works the best, as long as the stern lift is a constant, prop diameter below the hull plane creating horizontal thrust and prop diameter above the hull plane creating vertical stern lift.

Thanks for all the info. I tracked down some 1" spacers. I'm going to try some 34 pitch 17" 5 blades as soon as I can and then if the slip is still higher than I like ill throw the spacers in it. I honestly don't care much about slip I just don't want it blowing out the props at 100 in smooth water.

I didn't think you wanted to try spacers.
But, for future reference, we make billet aluminum spacer plates and we have a test program so you can try them without have to purchase them. We make them in 1/4" increments.

I don't think we ever actually heard what the actual depth of your prop shaft below the bottom of the boat is. Also, what is your rpm and speed at WOT? Let's get a actual slip #.

Brett

Not enough diameter.

I went back and saw the prop shafts are 1.5" above the bottom.I believe the spacers are going to help quite a bit. 17-17.5" is plenty of diameter for what I believe to be a 42 Fountain.
Still would like to hear some actual performance numbers.

Brett

You have spacers for Ssm 4's? Or are you thinking he has 6's?

Stepped 42 fountain doesn't need as much diameter as big heavy cig.

Oops, I have #6's.

Thank you,
Brett

21% slip at 106. If it was just a matter of slip I would just run larger pitch props and let it slip. I don't care for them breaking loose before they should.

Have you checked the hull for straightness, hook???, sharp trailing edges???
If you get on plane OK with the props you have but blow out at top speed - sounds like something else might be wrong.
Drive toe in or out depending on prop rotation?
Wrong style props?

How high are you trimming it at top speed. Do you have a cable indicator? What # is neutral, What # for top speed?

Can't really use the trim at all and it does not blow out on flat water. Glass smooth water and running over a boat wake will sometimes get them to break loose. It's hard to describe but I can't tell they are right on the edge of traction. That's why I feel like I'm close and going to try the other props before the spacers. Also the props I have been trying don't have much cup out by the tip and I know that can cause some of this.

What are you running to slip at 21%, 4 blade props with shorty drives, that's embarrassing for a 42 lightning, if so, space down, pitch down, and blade up.

did you read any of the thread? He has Ssm 4's on it. No shorties for those. And 21% is only about 5-6% off what I would expect for that boat.

With my heavy ass cig my slip is 11%.
They are too far out of the water, I bet the rooster is impressive. I`ve tried about a dozen props, and 2" spacers and switched between in/out a couple times over a 4 year period. That would be my advice.

17" of diameter should be fine for this hull. Putting the propellers 1" deeper in the water should only help, not hurt anything.
Additionally, having the propeller gone through and face lifted could help greatly.
Also, depending on what is in the cabin, your CG may be slightly forward.

Let us know if we can help with your propellers. I'd love the ability to discuss this wit you.

Brett

Brett gave you another option here, drive down, diameter up, pitch down.

What motors are in the boat???

Procharged 540's. I'd say somewhere slightly north of 900's.

Carburated, MPI, EFI, engine weight per side.

My fountain is a totally different hull than your 42 Eric, but, from what I have seen, Fountains, don't need stern lift. The Pad helps keep the azz end of the boat up on top of the water. However, it is a 42FT long boat. They need to be able to carry the bow. I think its a fine line of keeping the drives deep enough to have bow carrying leverage, but not too deep, to where its killing speed from drag from the lowers and prop.

My propshafts are 1.5 to 2" below the bottom. I honestly forget, its been a while since I measured. Anyhow, its a side by side 38 with #4's. I can trim the drives out to 3, then 4, then 5, and the more trim I give it, the faster it goes. I've trimmed all the way to 6 on the fountain indicator panel, and still no signs of blowing the props out at top speed, with 17" four blades.

I'd say if you are unable to run with any positive trim before blowing the props out, your drives are just too high. If it was a poker run 42 with no cabin and super light layup, maybe that wouldn't be the case. But with a pleasure version, heavy cabin, I think you'll find the boat might be more efficient and simply run better by spacing it down a bit.

Just my theory, and it may be way off.

https://www.youtube.com/watch?v=h3_jJR_TydA That rooster might be a bit excessive for a #4 drive full pleasure 42 lol

Were you trimmed out here? Or nuetral trim, or what?

Is this a stager set up? IF not why do I only see one roost. I always had two distinct roosts. Are you blowing out both drives or just one.
Sounds to me like you need to go down a little.

19.5" stagger so they are fairly close. Joe that is only trimmed up slightly. I have not tried to trim up that far because the slip is already high and it has been running good numbers. I was pretty thin on prop testing time since the season was over so didn't really get to experiment much.
Motors are Holley efi. Not sure on weight.

How does she carry at high speed? Does it seem like its running wet ?

Wonder where the 900sc #6 stagger 42s were for PS height?

like I said huge roost cause the drive are too high and/or prop is too small (for that height.)
A rooster is just wasted energy.

Mild Thunder has it right 2 inches under the hull is where the shaft should be especially on staggered motors, that's standard, 80% prop below the hull thrust and 20% prop above the hull lift, even to the hull, is actually -2, 67% prop below the hull thrust and 33% prop above the hull lift, with a 17 inch prop, that's a 65% increase in stern lift that the 42 lightning does need with the staggered motors and 5-blade props. For mine at 67/33 a 4-blade slips 23.39 and a 5-blade slips 36.64, because the 5-blade has 25% more lift than the 4-blade, at 80/20 the 4-blade slips at 18.71% and the 5-blade slips at 11.45%, currently I run at 86/14, a 4-blade will blow out and not stay on plane and the 5-blade slips at 8.97%, that is what I look for when fitting a prop when one less prop won't stay on plane and blows out.

Well joes is a straight bottom side by side so I'm certain he and I would require different xdim. Even when there were no 5 blades avail my boat would have never been rigged that low.