Tahoe 23 prop size?

[Brad Tucker]

Brad

Did you add the Ice Chest?

Bill

SEE???? I SAID I MISSED SOMETHING!!!!

[Brad Tucker]

I emailed the place I mentioned and they recommended a 13x11 based on the weight range I mentioned above. That sure seems outside what we have discussed here. He said for the Boat"s weight I needed as much diameter as I could get under the boat. I am not convinced after the discussion here. I was hoping they would recommend one in the same range as we have discussed. Am I missing something here?

[kens]

most of the data in this post is flawed,
I read a target of 50mph, with a 17 pitch prop, and that same 50mph required 268hp at the wheel.
According to your opening comment, you aint got 268hp to give.
You stated 240hp at the engine, then take away losses, you might get 225hp on the wheel.
At your available HP, you are gonna have to accept something like 45 or 42mph.
Given this HP and speed, a 17 pitch sounds heavy.
Dont forget that almost ALL of the wheels in this ski-boat size range are heavily cupped.
A cupped wheel @ 11 pitch, will run like a 13 pitch un-cupped.
A cupped wheel @ 13 pitch can run like a 15 pitch un-cupped.

Correct me if I am wrong, but you made reference to Barto's boat for weight??
If that is true, then you have to run a flatter pitch with a 1:1 gear. (for a given engine installation, as weight goes up, pitch goes down)
Notice that the standard Riverias & Monacos run approx 13x13 or 13x15.
If you upp the boat to Barto's size, then you have to flatten pitch, maybe 14x11, or 14x13
Install as much diameter as you can

[Brad Tucker]

If a 13x11 in today's standards will run like a 13x13 traditional due to cup, it makes sense. That is why I asked here, because I know little about hydrodynamics. I'm a flight medic, not an engineer.

In the thread, 50mph was plugged in initially as a reference because I didn't state one, as I'm more interested in WOT being 4400 rpm, and I am sure it's more complicated than that.

I'm no spreadsheet guru, but the one used appears to take into account hp loss at the prop vs BHP rating. I dialed down the speed (it was my main variable) until the shaft HP matched the needed shaft HP, and it ended up being 39-42 mph, based on weight differences using a 12x14 or 15.

In all reality, I was initially going to start with a 13x13 and go from there, but I wanted to ask here first. Since there are no Tahoe23 boats that have been built with my particular engine I'm aware of that have been weighed, and only 1 that has been weighed (Butch), I took his weight, estimated the weight needed to subtract. I did that by dividing his weight by 24.5, then multiplying that by 1.5 to roughly estimate the weight on average of a 1.5' cross section. I then roughly estimated the weight of an added layer of 5mm ply based on listed weight per square foot. Then, I took the difference in weight of a 454 and 351. I subtracted those figures from the weight of his boat. Roughly, that should be closer than a SWAG (Sophisticated Wild ___ Guess) which is 3400 lbs empty. Then I came up with 2 weights: my wife and I with 1/2 bag of gas, and another 600 lbs for a few more people. The recommended props came out to a 12x15 and 12x14.

If a 13x11 cupped to whatever standard is equivalent to a 13x13 plain prop, perhaps I should start there. The cupped vs uncupped is good information and I appreciate it. Just need a good starting point.

[PeterG]

Really need the design displacement to get a starting number for the weight of the boat. Using the Simpsons rule per the Glen L article should get you a good figure based on the shape of the boat and the design waterline. I would have ballparked your boat displacement/weight at about 2500 lbs based on similar designs. I calculated a Key Largo at 20' to be 2200 lbs and my Malahini at about 1350 lbs using the Simpsons rule and design waterline. The calculated weight is a figure to use in making sure you don't go overweight in your build, something to avoid or suffer degraded performance. The Propeller Handbook by David Gerr has everything you ever wanted to know about powering a boat and determining prop size and pitch, worth a buy. It's very clearly written, by a guy who has a great history and lots of knowledge in boat design.

[Bill Edmundson]

Brad

At these weights and HP, I'm seeing 40+/-. And, a 12x15. At about 4000 rpm the blade tip on a 12" prop is doing around 209 mph. If the blade is not perfect you can start getting tip cavitation. At the same rpm a 13" prop tip is doing 227 mph. The guy's 13x11 will never get you to 40. It will come out of the hole though.

Bill

[Brad Tucker]

Thanks, Bill. I thought the same thing. It would leap out of the hole, but top end would suffer.

I'm sure she weighs more than 2200. Hull weight is listed at 2000 if I recall. Based on reading old posts, I'd say that is weight at the flip. I did some forum research prior to asking. I will probably get a weight during the initial launch process and post it, but I need a decent initial prop at launch. Unfortunately, based on all the variables, there is not a displacement weight on the plans. I wish there were a number, but there isn't. The only number is "hull weight" of 2000 lbs. I lurk more than I post because I only post things I am sure of. But I'm pretty sure this boat weighs close to what I've stated, based on the info I've provided. If I am off, I'm WAY off. She weighs somewhere between your 19 and Butch's 24.5. His is shy of 4000lbs.

[NAMEngJS]

Brad,

I have not personally used it, but Delftship program has a free version that supposedly has some basic hydrostatics features. You should be able to take your offsets and create a model of the vessel and then get the displacement of the vessel at the water line. Adjusting the waterline up every few inches or so would allow you to see how much greater your weight would have to be to achieve another inch of immersion. conversely, if you say 4000 lbs would put you at a draft that is much deeper than your waterline then you would know you have a problem in your calculations.

Below is the print out of another Hydrostatic program "GHS" (General Hydrostatics) of some modifications I am working through. This is a 19'-0" vessel and the LCF draft would be around 1'-0". The image below is to show what some of the output might be.

columns from Left to right are LCF draft, displacement in lbs, longituninal center of buoyancy (from Forepeak), Vertical center of buoyancy (from baseline), Weight per inch of immersion, Longituninal center of floatation, Longitudinal metacentric height, and transverse metacentric height.

again I cant speak to all of the capabilities of Delftship but since it is free you may give it a try if you are concerned enough.

[kens]

If a 13x11 in today's standards will run like a 13x13 traditional due to cup, it makes sense. That is why I asked here, because I know little about hydrodynamics. I'm a flight medic, not an engineer.

In the thread, 50mph was plugged in initially as a reference because I didn't state one, as I'm more interested in WOT being 4400 rpm, and I am sure it's more complicated than that.

I'm no spreadsheet guru, but the one used appears to take into account hp loss at the prop vs BHP rating. I dialed down the speed (it was my main variable) until the shaft HP matched the needed shaft HP, and it ended up being 39-42 mph, based on weight differences using a 12x14 or 15.

In all reality, I was initially going to start with a 13x13 and go from there, but I wanted to ask here first. Since there are no Tahoe23 boats that have been built with my particular engine I'm aware of that have been weighed, and only 1 that has been weighed (Butch), I took his weight, estimated the weight needed to subtract. I did that by dividing his weight by 24.5, then multiplying that by 1.5 to roughly estimate the weight on average of a 1.5' cross section. I then roughly estimated the weight of an added layer of 5mm ply based on listed weight per square foot. Then, I took the difference in weight of a 454 and 351. I subtracted those figures from the weight of his boat. Roughly, that should be closer than a SWAG (Sophisticated Wild ___ Guess) which is 3400 lbs empty. Then I came up with 2 weights: my wife and I with 1/2 bag of gas, and another 600 lbs for a few more people. The recommended props came out to a 12x15 and 12x14.

If a 13x11 cupped to whatever standard is equivalent to a 13x13 plain prop, perhaps I should start there. The cupped vs uncupped is good information and I appreciate it. Just need a good starting point.

I agree with your thinking here.
however, I would go with the largest blade area I could get.
That will eliminate all the 12" diameter props, and steer you to a 13" dia 4 blade or 14" 3blade.
I say that because Barto and I have approx the same size boats, he is running a 14" 4blade, and I like a 16" 3 blade.
Your thinking of a 12" is not even close to either of us.
My thoughts are that you stated you are strut sized for 13".
Remember that the great many Monacos & Riviera's run approx a 13x13 3blade with a similar engine.
Your build is a larger hull with similar engine to the smaller Monaco.
Therefore, as you go up in hull size, you upsize the blade area, up size to 4blade.
Your math is believeable at 42mph, so you pitch to about 42mph.
My final thought is a 13" dia 4blade x 12 pitch (with cupping)

[Rich Coey]

I agree with Kens on this. My Monaco is lighter and I have more than 240 HP and my WOT is closer to 5000 rpms. I think the stats calling for 14 or 15 pitch are too high. Most of the time I am running an Acme 13 x 12.625 .080 cup 3 blade. When we are pulling tubes or skiers I switch to a 13 x11 .120 cup.
If it was an Acme prop I would recommend the 13 x 11, other brands 13 x13. Acme claims their 3 blades have as much surface as others 4 blades, with a hole shot like a 4 blade and a higher top end. With your specs you are probably looking at 40 to 44 mph max.

Here are pictures to show the difference in surface area.

OJ Legend

images-2.jpeg (4.91 KiB) Viewed 6229 times

Acme

images.jpeg (5.53 KiB) Viewed 6229 times

[Brad Tucker]

Thank you for all of the information. I really do appreciate it!

[Bill Edmundson]

Brad

From the other info you can probably go with a 13" prop. But, you will have to have 15" pitch at least to get to 40 mph.

Bill

[Brad Tucker]

After talking to Kevin at Acme, I'm starting with a 541 (13x12) and going from there. It seems less than what I would have thought, but the 541 Acme apparently performs like a 13x13 old style. I will post the final pitch here, so it may help future builders with a similar setup.

[Brad Tucker]

After running the 13x12, I found that it would allow the engine to exceed the max WOT of 4400rpm. I'm not sure by how much, because I didn't allow it to happen. I had more throttle left, and was running mid-upper 30's mph at 4400 rpm. Test run 2 was with an Acme 13x13. Max attainable rpm was 4200 with 42mph. Interestingly, I was getting around 28mph at 2500 rpm. The Speedo is a Faria GPS Speedo and I did not cross check it with any other GPS device. I'm thinking I will keep her propped with the 13x13 Acme, although the 13x12.625 they offer may be the perfect prop. Engine is a mid 90's PCM 351 rated at 240hp with very few hours.

[Rich Coey]

The 13 x 12.625 is what I use most of the time. Before that I had the 13 x 13 and like you it was holding the rpms down a bit. Switching to the 12 x 12.625, the top end will probably be about the same, but would be a little better out of the hole.

Rich