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MEGASLAM

Hydrofoil

25 posts in this topic

I've read a couple threads on the board and it looks like the genernal opionon is they're offsetting a prop issue.

However, I still want to ask a couple of questions. My current prop is working OK for fishining purposes but for family purposes I want to have a solid plane at lower speeds - say 10-15mph. I have a 19 crestliner tournament with 200hp is this even possible should be the first question? Should I continue (my never ending quest for the perfect prop if you've seen my other posts) to play with props and maybe increase the diameter?

All posts welcome!!!

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Let's see,.... A 19' Crestliner with a 200 hp, sounds like a large, wide boat with plenty of planing surface and a fair amount of weight up front. That's a good place to start. At what speed can you maintain level plane now? Also, why do you want the 10-15 MPH planing speed? I would not mess with the prop here since you will have more than enough power with the combination that you have. Change one variable at a time.

If you are pulling kids on tubes, then the extra leverage that a hydrofoil gives at low speeds will be a good addition to your combination. Trimming the motor down will put that whale tail into the water, levering the front of the boat down, enabling you to hold plane at lower speeds than you can now. Most likely your motor is set high enough so that at WOT speeds the cavitation plate is out of the water where the hydrofoil will have no effect. In that case it is the best of both worlds. I would say give it a try!

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Can you tell us what your RPM's are at Wide Open Throttle, trimmed out?

Adding the hydrafoil won't hurt, but IMHO, you probably don't need it on this big of a boat.

marine_man

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What I haven't quite figured out is this. If adding the wider surface on the lower unit is better, then why don't the manufacturer's design their outboards with the cavitation plate wide already?

Seems to me that if you've got the right HP for the boat, the extra wide plates won't gain enough to be worth the expense.

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Hi marine_man, sittin about 5600 at WOT with a 13 3/4 19 pitch "black stainless" Still think I need some more stern lift overall because I can only still slightly trim it out at WOT and I've never found a situation where my bow is too low. I only gain a few mph trimming it out. It still feels like I'm ridding the wave too much instead of powering through them when I'm trimmed all the way in. I think I should of stressed the OK part in my original post because I think I can still find even better perfomance. If I went up to 14.5 diameter what is the corresponding change I would need to make in pitch to mainatain (or slightly increase) WOT? I haven't found any calculators for this and was going to rely on the dealer.

I had the same thought BobT. Why wouldn't they just make the plate larger if it leads to better performance! Seems like I won't hurt anything by adding it but it is a commitment if I start drilling holes.

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I will say that I had a 16' Lund Rebel with a 25hp on it and I was under-powered. Adding the Hydrofoil did help in that situation by helping improve hole-shot and allowing me to trim out one notch but after upgrading to the 40hp, I got the impression they wouldn't provide any advantage anymore. The boat was rated for 45hp.

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I agree with your sentiment here Bob... they do provide a benefit on "under powered" boats, or low hp motors to get on plane quicker. Again, in my opinion, it's kind of a band aid for a propping problem.

Megaslam - where is your motor mounted on your boat?

marine_man

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Below is a picture of how she lines up with the cavatation plate. The motor is on the second hole from the top. Looks good to me or at least I hope. There is no way its being moved without professional help. What do you think?

As I write this, I guess a guy could take a 6 foot straight edge to see how well they line up? What should I look for?

DSC02779.JPG

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Looks to me like the outboard has a short shaft when it needs a long shaft. Shouldn't the cavitation plate be located about level with the bottom of the boat or perhaps even about 3/4" below?

With it up that high, I suspect he's not getting any lift because the plate is out of the water. Once up to full speed and on plane, maybe you could get by with this but to get there may be a problem. Correct me if I'm wrong.

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At that angle its hard to tell. It certainly isnt off by 5 inches, which is what the difference is between a short and long shaft. The plate is actually an "anti-ventilation" plate. It's main purpose is to prevent air from being pulled down from the surface and into the prop blades, ie, ventilation. It may aid in stabilization, but thats not its main function. If it were, then as you mentioned in an earlier post, it would be wider.

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It looks to me like it's pretty close as well.. maybe a touch high when you tilt it up, but not too bad.

I would take a straight edge and see where it really is, but I think it's fine, or no worse than 50% of the other boats out there now.

marine_man

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Sure looks like it could be a short shaft...I know it's tough to tell from the pic, but that was my first thought....5 " too short.

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I had the same thought BobT. Why wouldn't they just make the plate larger if it leads to better performance! Seems like I won't hurt anything by adding it but it is a commitment if I start drilling holes.

Working in the die casting and injection mold industry I can say, It would be almost impossible to produce (in mass quanities) that large of a fin. It wouldn't fill right, would be filled with tons of voids, and be extremely brittle. The size and cost of the mold to produce it would be insane and I can almost guaranty you that fin will break off inside the mold on a regular basis (meaning LOTS of downtime). Believe me, most of the motor companies have probably looked into it. Its just not feasible to do it at a cost standpoint when the consumer can buy an aftermarket hydrofoil for pennies compared to what the motor costs.

And looking at the picture, it looks like the motor may be about 1 inch higher than recomended, nothing I would be overly concerned about.

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Looks like both 19' Tournaments on Crestliner's webpage have 25" transoms..

Is that a 200hp Yamaha 4 Stroke?

According to yamaha's webpage, they offer that motor (now anyway) in a 25" and 30" shaft only...

So, that said, there's a very small / slim chance that you have anything shorter than 25"

marine_man

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Thanks for all the help guys! This is my first boat so its amazing what you "don't know you don't know" when joining the boat owner ranks. Its definitely NOT like buying a car or should it be.

Thanks for the research marine_man. I would be surprised to be that far off and have the boat function at all. Yep, its a 200.

Here's another picture to chew on. I've added a 4' level as a point of refernce. It is running parallel about 1.5" from the bottom of the boat next to the keel. The motor is trimmed all the way in. The boat is at an awkard angle on the driveway so I hope that doesn't skew the picture for you.

Couple of other questions came to mind (after a couple of lite's) staring at this angle. Why doesn't the keel run all the way to the back of the boat? What purpose do those two other horizontal "fins" serve on the motor and does it effect where the motor should be mounted?

IMG_0265.JPG

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OK, I'll admit my bad perception of it being 5" too short, sure looks closer now.

Marine Man can probably answer re: keeo all the way to the back of the boat, I though it was flatened out closer to the transom to allow for a cleaner flow of water to the lower unit/prop.

My Lund Fisherman flattens out too, and I think alot do.

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Hemlock is right on with respect to the keel.

I'm not 100% certain what the remainder of the flat fins are for.. but, bottom line, the cavitation plate should line up with the bottom of the boat.

Yours isn't that far off imho, but if you're in the mood to tinker, try lowering the motor one hole.

Now, is this a 200 Hp Yamaha 4 Stroke? If so, and you're turning 5600 rpm, you're about 400 rpm short of where it should be running, alluding to needing a slightly flatter prop. which seems odd given the boat / motor combination.

marine_man

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Yep its a 200 4 stroke. It runs in the mid 50s mph at WOT slightly trimmed out with me, a light amount of gear, and a half tank of gas if that helps indicate a performance issue.

Also, I know the lower unit was replaced before I took possesion. I would guess a thousand things could go wrong after a major procedure but the motor runs like a top but I'm still looking to get a more solid plane out of it. There were some other mods done that will shift the weight around such as it was converted from a sports console to a walk through and a bench was added above the live well in back. But that was all done before inital delivery (back in 2003) so i would think the motor/prop would of already been optimized. I think its getting out of the hole fine with the current prop. oh yeah - It also handles more to my liking when I have a couple of suit cases and coolers in the bow.

I'm learning prop selection is more of an art than a science. Do you think going to a 14.25 or a 14.5 diameter would be the next thing to play with? If so, how much should I change the pitch (keeping in mind that I have a 13 3/4 19 right now that with my WOT I could easily go to 18)?

I hate to admit it but maybe I should bite the bullet and have someone at a marina take it for a spin just to see if my expectations are realistic. (man, that's worse than asking for directions)

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The other two horizontal fins are pretty much just splash guards, nothing to affect performance.

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At this point I'd try whatever you can get your hands on to see how the boat performs. I think I would try a same pitch prop, but with a bigger diameter to isolate what impact that has on your boat, then, if need be, try something with a little less pitch (like the 18). The main thing is to try and change one variable at a time, otherwise you'll never know what had the greatest impact.

The other thing I would do is talk to Soderblooms about what you have and what you're after and see what recommendations they have for you. Many FM'ers have used them and been satisfied.

marine_man

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I have a comment here about the original question of how to stay on plane at slow speeds. It seems that the discussion has shifted to top end performance and that is a very different subject. From your pictures, the motor is bolted directly to the transom and not easy to move up and down, so I would leave it where it is. The cavitation plate is just above the hull bottom and that is deep enough for what you have. If you can hit the "mid 50's" with your combination, you are propped and set up fairly well for top end and probably about all you are going to get with that type of hull.

Now if you want to keep the boat on plane at slower speeds for "family activities" I would again look at your original idea of the hydrofoil fin. Keep the prop and setup you have, but use the tilt to trim the motor way in. That will put the hydrofoil down into the water, forcing the stern up. You will get the same effect as putting weight in the front of the boat. At slow speeds, the extra leverage provided by the larger plate behind the propeller will give you the lift needed to stay on plane at the low speeds you are looking for.

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Isn't staying on plane and staying level two different things? I wonder if it will help him stay on plane or just keep the bow down, which could mean plowing more water.

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I think the discussion is great! We meandered a little off the subject topic but you guys have given me plenty to think about. Keep it coming!!!

Did some more research into the foil and it looks there is a non drilling option that may be a cheaper experiment. I still want to toy with a larger prop because the majority of configurations now ship differently then the way I'm setup. I'll report back on what I find.

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I'd look at a four or five blade prop like Yamaha's SS Performance 4 blade 13 3/4 x 18 or something in the new High Seas series. Mercury also makes the four blade Trophy and a five blade High Five.

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With regard to "staying on plane" vs. "staying level", look at it like this; The size and shape of the hull is a constant that can be thought of as "planing area". By using trim and a hydrofoil you will can control the boat's angle of attack to the water to be a constant. The viscosity and compressibility of water make it's resistance to displacement increase with speed at a predictable and somewhat constant rate, The forward speed is the variable in question.

By using the hydrofoil trimmed down and keeping the angle of attack close to level, as the boat's forward speed increases, the water has increasing resistance to being displaced, forcing the hull upwards. At some speed it will rise to the point where it is more or less up on top of the water and "planing". When this happens, the force required to move the boat forward is reduced by not having to move that water out of the way. You will have to experiment with trim settings as you increase speed, and to hold the boat on plane at lower speeds. Your minimum planing speed will be determined by playing with both trim and throttle simultaneously.

The principle here is to use the maximum planing area available in the hull design to displace the water and keep the boat on top of the water surface. It uses horsepower and you will not go very fast doing this, but you will stay on plane at a lower speed. Also, the comment above about the 4 blade prop will help with the low speed part and hole shot part of the equation, but 4 blade props often have a trade off of a few MPH on the top end.

Enjoy the experiment!

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