How To Select a Prop (it's not magic)

[hwsiii]

Re: How To Select a Prop (it's not magic)

LOL Gary, I have a new keyboard already ordered for my notebook. But I appreiate the kind thought and I think it is kind of fun seeing if y'all an figure what words I am typing.

And my head ertainly won't explode as I have ALZHEIMER'S and I forget it as soon as I do it. LOL



H

 

[wca_tim]

Re: How To Select a Prop (it's not magic)

If you really want to d oset up right from basics, you need to include gear ratio as a variable. often times the factory gear for a given set up isn't where it should be to make use of the mest performing props on a given hull, weight, power combination. ideally you want props in the pitch = 2x diameter range, or on the order of 1.8 - 2. especially once max speeds get much over 50 mph. well designed stainless props in that ratio range have the optimum amount of slip and angle of attack to convert rotational torque into forward thrust. for the absolute optimum, you would have the minimum blade area as possible to avoid wasting energy turning more blade than you need and the minimum diameter to do the same plus prevent cavitation out of the hole. everyone knows fpr the power and prop shaft rpm that most pleasure boaters use, 13 - 15 or so inch props are the ticket. that puts us in a pitch range of 26 - 30 for best performance... why do most outdrives for i/o drives have gear ratios that put people in the 19-23 inch pitch range? because most boats go slower than 50 and because id helps decrease the stress on the lower gearcase...

one of the keys is that the plot posted above from the merc about props page... changes as a function of the forward speed range being considered. it is an example, not an absolute thats why there aren't any units on the plot axes.

then come all the other variables... as in how narrow is the cone of rearward thrust? which is dictated by the blade geometry... and how does that impact the way the hull is carried at a given speed... too many variables... head hurts... think I'll just go try another few props and go from there...

so if I have a light boat, and i want to go fast, I should put the gear ratio where I would estimate a 27-30" prop will hit max rpm at wot... yes?

 

[hwsiii]

Re: How To Select a Prop (it's not magic)

Tim, that has been the consensus over the years is to run the prop with twice as much pitch as it has diameter. I will give you a tip that most people don't know for really high speed boats and it is a pain in the butt to do, but is very inexpensive, and is good for about 2 to 5 miles per hour depending on how bad the transition is now and the speeds the boat attains. Where the bottom of the boat meets the transom and the chines needs to be absolutely a clean 90 degree transition, and most commerialy built hulls do not have that, they are slightly rounded whih increases the drag a lot more than most people realize. If you make an absolute clean brake the drag is very muh improved. Hull design very muh influences the final results, doing the strakes an help, but it is more influenced by the transom first, then the chines in most hulls and lastly the strakes. The reason is beause it is more important to have clean transitions where the water leaves the boat than it is where the water just runs down the strakes.
Think of it like a prop, the cup is placed on a prop for one reason only, that is to hold the water better so it reduces slip and the same thing happens on a boat at the transom and the chines. On a boat they are not cupped outward like a prop is but it still creates that extra drag if it does not have a lean transition. If you decided to do this you would need to grind the gelcoat down and just make a thik epoxy with fillers to create these, you can't use fiberglass tape or any type of cloth for this because they can't make 90 degree corners, they have to be rounded corners.

That video is fantastic Tim and that setup sure sounds good.


H

 

[kick bass]

Re: How To Select a Prop (it's not magic)

Tim and Mr H,
You two are getting really deep. As soon as I can, I want to post more formula's. Tim is right about gear case design. If a Prop manufactor built one prop for one boat and one engine, you would not need a formula. The problem is cost factors. The same with gear case ratio. the problem is, both have to fit many boats. I like what you talked about in the design of the boat.
If an engine manufactor built a different engine for each boat, then they could tweek the gear ratio, the prop people could tweek the prop. They would both go broke. Boat and prop engineers love boats. But, their accountants love calculators. So, you have to have a happy medium.
Now for the hull design. Hull on water equals friction. friction equals brake. But, To little boat in water is not stable. the design of a hull like Mr H said can be used for the good. You can actually use water as a lubricant. Then what happens to the water after it leaves the boat is important, it effects the prop. believe it or not hull design sometimes is not set by engineers, but by what people want. Give people what they want even if it could be better. In the 70's everybody in the boat business knew a V hull Bass boat was faster than a tri-hull. but everbody built a tri hull boat bass boat. People wanted them. I know I will get posts on how much better tri hull boats are, if you like them run them. But a V bottom is faster, turns better and is better all around and that is why the boat people all use them now.
As far as props go, they are constantly changing. You see words like improved or best, in prop advertising. sometimes they put a two on the end. after all if you bought a prop, and it could not be improved on, everyone would have one, prop people go out of business.
I will post rake and cup next, just want to do it with out causing to much confusion.

 

[wca_tim]

Re: How To Select a Prop (it's not magic)

my current limitation in speed with a merc alpha drive is blowout... have hit a wall about 77 mph right now.

and to highlight the gear ratio thing... The little boat I keep screwing around with from a speed / performance standpoint is 3-5 mph faster running a 1.81 upper gear set and 27-29" props even then smaller diameter / lower blade area 21-24" props with a 1.47 gearset I have both. Next step is to build one of my spare uppers with a 1.65 and see if I've got enough torque at 5200 rpms to turn a 28 or 29" prop there. the hull has a pad, lifting strakes and steps...

and Mr. H is dead on about cleaning up the lines on the hull... tight corners on pads, strakes, transom, steps along with blueprinting the last couple of feet of hull (using a straight edge to sand out or fill any deviations from completely flat on the pad and bottom of strakes, etc...). Another thing that is really important that many neglect, is the cleaning up the running surface of the outdrive - When I reworked the current lower on the boat, I used a grinder first and then 40 grit paper on a grinder before feathering everything out to make some "minor" changes in skeg shape and to clean up the gearcase, etc... using layout blue and / or diluted black laquer paint as a guide and long sanding blocks to find the low and high spots along the contours. It seems to have made more of a difference than I thought...

unless someone is really chasing performance, the fairly straightforward and simple rules of thumb included in your posts above are the way to go... and for many, the next step up is to call one of the well known performance orianted prop shops... (I try the things i do because I enjoy playing with it, if I really wanted a race boat, I would buy a race boat rather than play with the one I have, I do this because it is a relatively inexpensive way for me to learn and experiment... and keeps me out of trouble)

 

[kick bass]

Re: How To Select a Prop (it's not magic)

I am impressed!
If any of you want to check on props. click on one on top of the page. it will give you an idea. before you click the BUY button, contack the and see what they say.