Prop Maximization.-

[Sea Rider]

For the experts, scenario ;

450 Rib
Deep V hull
400 Kg : Rib, driver, engine, fuel (minimum loaded)
Engine Tohatsu 2 stroke 18 HP engine
Well trimmed and deck ballanced
Wot rpm range : 5200-5800

Data :

To maintain plane 20 Km/H
Cruise speed : 35 Km/H 4.5K rpm
Wot : 5600 rpm
Wot speed : 41.5 Km/H
Delivered Prop : 9.2 x 9.1 (9 pitch)
Flat glassy no wind water cond.

2 stages to plane, 1/2 throttle untill Rib is on its way, 3/4 throttle to plane with just driver.
This sib is rated for 7 passengers and 50 HP engine.

Usually go boating with min 2 up, max 3 up driver included. So no need a larger HP engine, would like a prop maximization.

If going 1 less pitch to 8, (9.2 x 7.8 ) theoretically should pull up my revs + 300 revs to be at 5,900 ( slight over rev.no big deal. Max wot rpm will dercease accodingly as soon start carrying more weight or add boaters aside from driver.

Question :Will my hole shot improve a bit along my top wot speed if revving at 5.900 solo ? Usualy like running at cruise speed on flat calm days, but like having my horses running near full wot rpm factory stated.

There's a 8.5 pitch alternative (9.2 x 8.8) that should put me around 5,750 rpm. Should I choose the 8.5 or 8 pitch prop for best top performance ?

Happy Boating

 

[Texasmark]

Rule of thumb is about half or a little more per inch of pitch change of what you said about revs. Diameter is not that big of a deal, usually varies as the design of the prop changes to get the desired pitch and changes with different mfgrs. designs and types of props.

Agree that running over the limit on 2 strokes is no big deal. Have done it for years and my 2 strokers loved to rev and run.

Any decrease in pitch will improve the hole shot and maintaining speed at higher loads and rougher water.

Speed reduction vs pitch change is dependent on hull/load and engine rpm increase. If things go your way you won't have any reduction as the increased rpm might make up for it. Otherwise there will be a slight decrease that you can calculate: Pitch of the prop x rpm x slip (15% is a good number for a medium fast hull) vs 5280ft/mile per hour.

Let's look at some numbers:

A 9" pitch x 2800 rpm (assuming 2:1 gear ratio with engine turning 5600) x .85 (15% prop slip) = 21420 inches per minute forward travel of your boat. 1 mph = 5280 ft./statute mile x 12 in. = 63360 inches per hour or 1056 inches per minute. Dividing prop forward movement/revolution in inches per minute by miles per minute = 20 mph. (32.2 Klicks)

Assuming nothing changed in boat "aquadynamics" including prop slip not changing, which usually move around any time you change something, looks like the 8" @ 6000 rpm will do: [(8x3000)x.85]/1056 = 19 mph. (30.5 Klicks)

Looking back at your current numbers, your speedometer is a little off (mine always read a little high the higher on the dial you were) or your prop slip is less than 15% which is great! Also for this to be realistic, the prop characteristics of the new one have to be identical to your current prop.

Edit: Oh, to clarify a point, I'm no expert. Just passing on related experiences, some book-work and 50+ years of enjoying the sport!

HTH,
Mark

 

[Sea Rider]

Thanks Mark

Gear reduction ratio for that horse is 1:85 (13:24) wot speed was measured with 2 Garmin GPS on 1 kilometer wot run on flat glassy no wind sea. The Rib has a fantastic hull design, Both mentioned 8.5-8 areTohatsu props, have same diam and design.

How much will 1:85 gear ratio change posted data previously calculated to 2:1 ? So 1 pitch increase/decrease is just 150-200 revs and not 250-300 ? If so a 8 pitch should put me right spot on 5800 max wot revs for that engine to give better hole shot and handle more load..

Happy Boating

 

[steelespike]

Using mph to calculate ; your speed is by gps so we can assume correct.Boat can't go 21.5 @ 4500 .Or 25.79 @ 5600.
If we use 12% slip and mph, I get 5245 @ 21.5 and 6292 @ 25.79 mph.
Usually slip is in low double digits.If slip turns out to be lower rpm would be lower.
I think you need to confirm your rpm.Is the 400 kg the dry weight?

 

[Texasmark]

Just divide the engine rpms by 1.85 rather than 2 and use the same math that I used. Your prop will be turning faster as it is closer to the engine rpms by 2.0/1.85 = 8%. The boat speed numbers I gave you will increase by 8% if nothing else changed in the process of the boat attempting to go that 8% faster.

Edit: You can calculate your slip if you have the rest of the numbers. Go to the www of go-fast.com and plug in the numbers in their prop slip calculator. For gear ratio just put the number 1.85. Leave blank the column titled slip. Punch the calculate button fort it and you'll have it. Then you can back track or play with the numbers to your heart's content to see what happens to the things listed. Remember, the boat's aquadynamics play into this and until you actually run the prop on a selected load under certain weather/water conditions you really don't know where you'll be.

Mark

 

[Sea Rider]

250 Kg for Rib, 100 for driver, 50 for engine/fuel makes 400 Kg. So going to the original question, need a 8.0 or 8.5 pitch prop to raise my wot rpm to 5800 ? Now running 5600 with 9 pitch. More speed is not main factor, better hole shot and top wot rpm is what I'm after.

Happy Boating

 

[Texasmark]

Drop the pitch to suit you. Less pitch will do what you want and how much depends on what you get with what you select. You have the information to make an "educated guess". You know, you don't have to firewall it if the boat is lightly loaded and wants to rev up to numbers that bother you. Save your WOT for the times when it is loaded, and for the hole shot, and you will do just fine.

Mark

 

[steelespike]

250 Kg for Rib, 100 for driver, 50 for engine/fuel makes 400 Kg. So going to the original question, need a 8.0 or 8.5 pitch prop to raise my wot rpm to 5800 ? Now running 5600 with 9 pitch. More speed is not main factor, better hole shot and top wot rpm is what I'm after.

Happy Boating

According to my calculations you wot rpm is about 6200 rpm. The boat can't go 41.5 Kmh at 5600 rpm.
If you go to a 8.0 or 8.5 You will be raising the rpm even more.Something like 200-300 more rpm.
At the least you need to confirm your actual rpm. Your motor is doing very well at 41.5 kmh/25.79 mph. But assuming 12% slip your actual rpm calculates at 6292.
If you want better hole shot you may want to try a 4 blade prop in the same pitch this may improve hole shot and lower the 6292 rpm a little.

 

[Sea Rider]

That same engine used to run 41.0 Km/h at 5790 rpm on previous 430 Rib which weighted 100 kilo less than actual 450. The non deep V hull was not that efficient as my actual deep V hull Rib. Have made solo wot trails on flat glassy non wind water cond with 2 induction tachs. Tiny Tach read 5600. Hardline Tach read 5595 rpm so accurate for both to read near even.



Tohatsu 18 HP is a world champion in its class, will add that water flow is slightly skimming under upper small water deflector plate as opposed to middle leg or around anticav plate.



My wake is darn flat due to having matched ideal sweet engine/transom height that accounts for top blade grip & thrust to get all the ponies out of that nice engine along impeccable tight turns on any water cond.



Will try a Tohatsu 8.0 pitch , must be here in some weeks, is coming factory direct. No experience whatsoever with 4 blade props, preffer to play with 3 blade props that have played with and don't throw money into the blue if a 4 blade doesn't dial what I'm after. Will post results...

Thanks you both for your kind imputs.

Happy Boating

 

[steelespike]

Perhaps the prop is cupped.
​The prop calculator I use suggests that, if a prop is cupped, to calculate, add 1" to the prop size also if I lower the slip to 10% we get a result around 5600 rpm.

 

[Sea Rider]

If by cupped you mean having a crown like at prop end, then yes. Is 10% slip a good parameter to have ?. What's the least % slip possible to have with any best set up ?

Happy Boating

 

[steelespike]

10% is a good slip number. Single digit slip is unlikely in a typical boat or setup. Boats that are hard to push
by weight or design will have higher slip.Usually as rpm goes down slip goes up. Getting a concrete slip number is a little tricky
but it is safe to say 10 to 15% is pretty typical for a planing hull.Less than 10 % would be initially suspect.If the slip gets over 15% with typical family runabout it may be time to evaluate the setup.
I'll be perfectly honest I'm a seat of the pants guy with props.To the best of my knowledge I've never had a cupped prop ion my hands.
Cupping can improve performance and depending on where it is applied may change how the hull responds.
If you have a factory prop it isn't likely cupped for a 18 hp Tohatsu.But that doesn't mean it's not efficient.

 

[Texasmark]

If by cupped you mean having a crown like at prop end, then yes. Is 10% slip a good parameter to have ?. What's the least % slip possible to have with any best set up ?

Happy Boating

If this is the Original Equipment Manufacturer (OEM) standard prop it is probably aluminum and probably NOT cupped, especially for an engine of that horsepower (HP). Few aluminum props are cupped and especially in your HP range. Look at the blade of your current prop and maybe even put your finger on it. Start at the root, where the blade connects to the outer hub, and follow the center of the blade out to the tip remaining in the center of the blade. If, once at the last 1/4" of the blade, the prop makes an abrupt turn up such that your finger would ride up and over it as compared to moving down the blade then your prop is in fact cupped.

So, back to what I said about "like kind" if you are going to swap props. If you change designs including cupping, high rake (blades sloped to the rear as you move out toward the end of the prop), or stainless steel (SS) which has thinner blades and are more efficient as a result, then you have to get a prop of like kind or the numbers could be way off.

On slip, IF you do have a cupped prop, it will slip less than a non cupped prop as it grips the water better, should be obvious to understand that by looking at it and how the blade slope changes at the tip, and as Steel said, it calculates as an additional 1" of prop pitch when running numbers measured at Wide Open Throttle (WOT). Therefore, to minimize your slip, you would do better with a cupped prop and one made from stainless steel. One nice thing about the cup is that it is not all that effective in the hole shot; designed for WOT gripping improvement. So you can go fast and have a better hole shot also since you could theoretically drop 1" of pitch and use the cup to get back your speed at WOT.

So, if you dropped to the 8" and current prop was NOT cupped but new one WAS, then you would have a better hole shot and the top end would be very close to where you are now.....best of both worlds. If you add SS to that you might pick up a mile per hour on your rig and if you bought the SS and it also had high rake (which I doubt again for that HP) then that could help in reducing your slip also and improve your ability to hold in turns.

Moving right along. Earlier you said that the water coming out from under your boat was just under the splash guard which is above the antiventilation plate by an inch or so. If you got this differently designed, hot dog prop, you might be able to move the engine up a notch) 3/4" higher on the transom usually with the same control in rough water and turns. This would be very helpful in reducing your "drag" and would/could give you another MPH or two. But, you said you didn't care much about WOT performance so forget that.

Mark

 

[Sea Rider]

Mark,

My standard factory delivered prop has a curved lip on the trailing edge same as the 8 pitch prop, so is it a cupped prop ?

Would be $$$ counterproductive to switch to a SS prop, it's just a 300 CC engine, have read that SS props should be installed on boats running + 50 miles per hour for best top performance, far away than my actual speed. Anyway Tohatsu does not sells SS props for that engine, just alum ones.

My engine is just 1 centimeter off height where I like to have it if boating solo, with more load or + 1 passenger hull lowers enough for water flow to skim slightly under small water deflector plate as in posted picture, Tohatsu engines loves this height setup which works best.

Happy Boating