Need help with prop selection and depth for a bit different type of hull.

kentuckydiesel

Seaman Apprentice
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Jul 11, 2012
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This past weekend, I bought a 23' aluminum walk-through type boat which was built using a 1961 Lone Star "Cruise Liner" hull. I had been looking for one of these for about 3 years. Just love the hull design. Warped Vee style...very sharp at the bow, almost flat at the stern. (BTW, I bet you can guess the favorite movie of the guy who built the boat :rolleyes: )

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I estimate the boat to weigh around 1500 lbs , including the twin 20" shaft 70hp Johnson outboards. It's a fairly dry running type hull and it gets up on plane very quickly.

Yesterday afternoon, the wife and I took it out on the Ohio river. The first time I went to plane out, I noticed the props were ventilating very badly. Unfortunately, the outboards are down as far as they can go on the transom. The only option I had left was to move the tilt pins 1 hole down (to the bottom hole), just to get the props in the water a little bit more. After that, the ventilation issue got better...plus we put two more people on the boat. It would only start to pull air if I pushed the engines past 4500-5000rpm/got above 20-22mph or so. (but I think I'm looking at around 40-45% prop slip at those rpms)

Two things I'm thinking about doing.

1) The engines are set pretty close to center, and in that area of the transom, it is around 21 1/2" to the bottom of the hull. If I move each engine out to either side of the transom, it should put them in an area where the transom is 20 1/2".

2)The current props are 13"dia x 19" pitch. No cupping...slightly bent in a few places. I'm thinking, since the twin 70s on this boat are pushing the 13x19 props to slip so much, that I should bump up to 21" pitch.


Would you guys think I could do well with cupped 21" pitch props? I'm actually hoping that they would work out, as my 1974 Johnson outboard manual says the 13x19s will give a maximum top speed of 28mph...which just isn't much (and according to a prop slip calculator, it is 32% slip???).

Also, should 20 1/2" of transom be alright for the 20" shaft engines...or am I going to have to modify the transom?

Thanks,

Phillip
 

Georgesalmon

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Apr 14, 2012
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Re: Need help with prop selection and depth for a bit different type of hull.

Are you measuring the transom vertically? Don't measure along the angle. Sounds to me like you need to go down in pitch not up. Whats the WOT operating range RPM for those engines?
 

kentuckydiesel

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Jul 11, 2012
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Re: Need help with prop selection and depth for a bit different type of hull.

Are you measuring the transom vertically? Don't measure along the angle. Sounds to me like you need to go down in pitch not up. Whats the WOT operating range RPM for those engines?

I'm measuring vertically.

The recommended cruising/heavy load range (houseboats and such) is 4500-5000. Performance/lighter load range is 5000-5500.

With this boat weighing in around 1500lbs (including the outboards), it is definitely in the "light load" area...especially with two 70s on it rather than one.

Basically, I am getting alot of prop slip, combined with occasional higher rpm/load ventilation. 20-25mph is an unacceptable top end. I have seen where this same hull (still set up as a cabin cruiser...heavier and more drag-air and water) was doing 35mph with a single 90hp outboard(which they didn't believe was propped with a high enough pitch)

Thanks,
Phillip
 

steelespike

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Apr 26, 2002
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19,069
Re: Need help with prop selection and depth for a bit different type of hull.

I'm not sure but I think you will find the actual length of your 20" motors is over 20".
From your description I think a pair of new Solas Amita props will solve some of your venting issues.
These props are much higher tech than your 1974 props.
If your tachs are original I would be suspicious of their accuracy.
My calculations indicate that a a 19" prop at 5500 rpm 10% slip will make about 36.8 mph.
Is the anti vent plate(just above the prop) about even with the bottom of the transom?
 

kentuckydiesel

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Re: Need help with prop selection and depth for a bit different type of hull.

Tachs are not original. Believe they were installed in the late 90s. Also, I believe the props to be newer than 1974 as I have a spare set that appear to be the original Johnson props, though they are 13 3/4 x 15. That said, the current 19" props aren't in great shape.

Anti vent plate is even or maybe just a little bit above the bottom of the transom.

One other thing I was noticing...This boat has some fairly wide riveted strakes on the hull which just happen to be directly in line with the props. Would strakes be bringing more disturbed water to the props than a smooth section of the hull between strakes?

Thanks,
Phillip
 

steelespike

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Re: Need help with prop selection and depth for a bit different type of hull.

No question the strakes/keels and rivets can disturb the water also can create some drag.
I wonder if you could move them slightly closer together.Spreading them too much the outside motor in a turn might have venting issues. In the old days they would suggest removing 8" to a foot off the keel and slope the end to avoid disturbing the prop.
And this is with a 15" transom and a 15" motor that was closer to 17" Of course prop technology was pretty basic.
The Solas Amita and Rubex props use "squeeze cast" technology for finer,stronger al props.
I'm pretty sure they also have some cup.
 

kentuckydiesel

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Re: Need help with prop selection and depth for a bit different type of hull.

They are only 25" apart right now. Wouldn't really want them any closer than that.

Thanks,
Phillip
 

jestor68

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Jun 12, 2012
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2,308
Re: Need help with prop selection and depth for a bit different type of hull.

Big keels directly in front of the props are a problem. The water coming off those keels is introducing air bubbles right into the props.

A good prop shop can add cupping to the blades of your current props to help eliminate the ventilation.

Since 4 blade props are more resistant to ventilation, it may help to stick a set of those on there.

The last option is to re power with a single motor.
 

kentuckydiesel

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Jul 11, 2012
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Re: Need help with prop selection and depth for a bit different type of hull.

When I bought the boat, the engines were angled with the props pointing away from each other. I did a little measuring and found out that it was quite a bit, so thinking this might be my issue, I adjusted the engines toward each other just a bit.

Went out yesterday evening with four people including myself on the boat. No more ventilation accept a slight amount on very hard turns, but now it recovers quickly.

That said, I'm still hitting 6000-6200rpm wide open with 4 people on board...so I'm thinking that I do need to go to a 21" prop.

-Phillip
 

steelespike

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Re: Need help with prop selection and depth for a bit different type of hull.

Do you have a gps speed to help get the idea how accurate the tachs are. Not likely they are both off unless that model
a had the same idiosyncrasies.If they are turning 6200 I think they would sound like they were coming apart.
The calculator says that with 10% slip speed would be about 41.Probably have to temper that a little with the twin setup.
The 70 prior to 86 is rated at 5000 with a 4500-5500 rpm range.
 

kentuckydiesel

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Jul 11, 2012
Messages
48
Re: Need help with prop selection and depth for a bit different type of hull.

Do you have a gps speed to help get the idea how accurate the tachs are. Not likely they are both off unless that model
a had the same idiosyncrasies.If they are turning 6200 I think they would sound like they were coming apart.
The calculator says that with 10% slip speed would be about 41.Probably have to temper that a little with the twin setup.
The 70 prior to 86 is rated at 5000 with a 4500-5500 rpm range.

They don't sound like they are coming apart, but they sure are wound out pretty good. I don't run them at that speed, but I did want to check WOT RPM to see what I needed to do prop wise.
I have been thinking of checking the rpm with a mechanical tach just to know for sure. Will probably give that a shot this evening.

When I checked it, I was running at about 4500-5000rpm. Boat sure did seem to take off between there and the 6200 or so RPM that I hit at WOT.

Thanks,
Phillip
 
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