Prop discussion

Designs for inboard or outboard power

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Hercdrvr
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Prop discussion

Post by Hercdrvr »

What pitch and diameter prop do you think will be best on my 20ft Barrelback, 285 hp with a 1.23:1 transmission? I have a 12x12 4 blade from the donor ski boat and a 14x14 3 blade.
Matt B

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Roberta
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Re: Prop discussion

Post by Roberta »

With the gear reduction transmission, I think the 14x14 will be a better choice. Hopefully you will have adequate clearance between the prop tip and hull bottom. 2" will work. Are you running a 1 inch or 1-1/8 inch shaft. Remember to keep distance between prop hub and strut at shaft diameter or less.

Roberta
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

Hercdrvr
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Re: Prop discussion

Post by Hercdrvr »

Roberta wrote:
Thu Apr 22, 2021 8:27 am
With the gear reduction transmission, I think the 14x14 will be a better choice. Hopefully you will have adequate clearance between the prop tip and hull bottom. 2" will work. Are you running a 1 inch or 1-1/8 inch shaft. Remember to keep distance between prop hub and strut at shaft diameter or less.

Roberta
It’s a 1” drive shaft. I’m good on prop to strut clearance, it has 3/4” shaft showing.. I’m not sure if a 14” diameter prop will have 2” clearance on the hull.
Matt B

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kens
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Re: Prop discussion

Post by kens »

what do you mean 3/4 shaft showing?
you mean between prop hub & back of strut?
one diameter there is accepted as max
I agree with 14x14 for initial sea trials
Oak..........the juice ain't worth the squeeze :D :shock: :o :)

Hercdrvr
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Re: Prop discussion

Post by Hercdrvr »

kens wrote:
Thu Apr 22, 2021 9:11 am
what do you mean 3/4 shaft showing?
you mean between prop hub & back of strut?
one diameter there is accepted as max
I agree with 14x14 for initial sea trials
Yes, prop hub to strut clearance is 3/4”.

Hercdrvr
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Re: Prop discussion

Post by Hercdrvr »

Why is the 4 blade 12x12 the wrong choice?
Matt B

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Roberta
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Re: Prop discussion

Post by Roberta »

I'm assuming you have those two props on hand. The 12x12 is way too small in pitch and diameter for 285 hp and a gear reduction transmission. Your speed will be greatly reduced and the engine will not be sufficiently loaded. Kinda like running a car in first gear. Props need to be sized such that the engine rpm is limited and won't exceed maximum rpm under full throttle. If clearance is an issue, you may need to try a 13" prop with a 15 pitch. Erosion of the hull bottom can occur with insufficient clearance from the prop.

Roberta
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

denbrlr
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Re: Prop discussion

Post by denbrlr »

Matt,

Acme Propellers has a big selection to choose from. They have a 13x15.5 (number 224) for a one inch shaft. You might have to work with them on the cup though. Other builders can chime in here but I know several folks had to work with their prop supplier on tweaking the cup.

Speaking of props, there are different hub lengths. Is a 2.75 inch standard for runabouts like the Barrelback and Monaco? For 1.125 inch shaft, Acme shows 2.75 inch, 3 inch, and 3.125 inch as options but it looks like 2.75 inch might be standard for a one inch shaft.

Lee
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Roberta
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Re: Prop discussion

Post by Roberta »

2.75" is the industry standard for a 1" prop shaft.

The amount of cupping will influence the performance of a prop.

Roberta
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

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kens
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Re: Prop discussion

Post by kens »

Roberta wrote:
Thu Apr 22, 2021 9:32 am
I'm assuming you have those two props on hand. The 12x12 is way too small in pitch and diameter for 285 hp and a gear reduction transmission. Your speed will be greatly reduced and the engine will not be sufficiently loaded. Kinda like running a car in first gear. Props need to be sized such that the engine rpm is limited and won't exceed maximum rpm under full throttle. If clearance is an issue, you may need to try a 13" prop with a 15 pitch. Erosion of the hull bottom can occur with insufficient clearance from the prop.

Roberta
We dont know that for a fact. it might work depending on blade shape, cupping and so forth.
What we do know is that the old standard SBC 260hp 1:1 gear in one of these forum boats turns about a 13x13 on average.
your 12x12 is probably too small, but you can try it. You coming to the Gathering for first launch? bring it with you.
My guess is the 14x14 is pretty close, you never know until you try it.
The prop erosion thing doesnt happen immediately, like taking a router to your hull, it happens over time, and initial sea trial aint enough time.
And if you had it going on you would have a vibration and the buzzing should tip you off.
Oak..........the juice ain't worth the squeeze :D :shock: :o :)

Hercdrvr
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Re: Prop discussion

Post by Hercdrvr »

Good discussion, thank you for all the input. I’ll start with 14x14.
Matt B

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Re: Prop discussion

Post by denbrlr »

Matt,

One thing to watch out for is how close the prop blades are to the hull. You ideally want more than a two inch gap. For a Monaco, the plans call for a 7.5 inch drop strut and either a 12 or 13 inch diameter prop. I don't know the drop of your strut but that would be something to look at.

Also with a 1.23:1 trans, you might want more pitch. Maybe 15 or 15.5. Just some things to consider.

Lee
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Monaco build YouTube playlist: https://www.youtube.com/channel/UCP4Edb ... /playlists

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Roberta
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Re: Prop discussion

Post by Roberta »

I was looking at OJ Force four blade props. They have a 12.5 x17 pitch, RH, 1" shaft and a series of 13" props in 13, 14, and 16 pitch all 1" shaft and RH. These could be considered for 1.23 ratio transmissions. Would give you better clearance than a 14" prop.

Acme has some good choices in the Sport prop selector in the 12.5 and 13 inch sizes.

I run an OJ Force four blade 13x13 RH prop on the Torpedo 1:1 transmission and 330 hp. Very smooth and powerful yet limits the engine to about 5000 rpm and 50 mph.

Roberta
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

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Bill Edmundson
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Re: Prop discussion

Post by Bill Edmundson »

How many rpm does the engine turn?

Bill
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Hercdrvr
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Re: Prop discussion

Post by Hercdrvr »

Bill Edmundson wrote:
Fri Apr 30, 2021 5:20 pm
How many rpm does the engine turn?

Bill
I guess about 5,000 rpm would be a good target for WOT rpm.
Matt B

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