Prop problem definitions

Texasmark

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Dec 20, 2005
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Texasmark,

A simple boating question : assuming you have AV plate even or slight lower with respect to lower keel, what do you expect to accomplish when combo is placed on water and you go for a wot spin ?

Happy Boating

Well sir, on the boat pictured, 17+ ft. alum bass boat, 90 hp stock Mercury, 24 P Ballistic SS prop, 250# of me, 3 batteries, 15 gal gas, fishing tackle, trolling motor, and all that stuff.........50 mph at 5800 rpm with 12% prop slip! Wake is in my Avatar. Pretty flat and not much of it if you ask me.

If you'll notice my WDP Way up there and AV just ABOVE the "pad" (flat center 10" of hull). At high trim angles and really hard turns at high rpms I get some blowout but tucking the trim or slowing down slightly corrects it. The Ballistic has a nice rake and good cup so it grips very well.

Seems you run boats like the Zodiac Brand and I think they run differently than hard cast hulls, but I don't know I never ran one. I don't know what Mr. AGCB is running. Don't think he said. If he'd post a picture takes from low and to the side showing the mounting height, like your picture herein, it'd help to understand reasoning for his question.

To have your WDP under the transom bottom you really have to be low, like a 20" engine mid section on a 15" transom. Agree that each boat engine combo is unique and has to be tweaked for best non ventilating performance.





 

Sea Rider

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Sep 20, 2008
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Your set-up has a spectacular powerful flat middke wake, same as mine running a much less HP. The difference between both set-ups is hydraulic versus manual trim. I'll bet you that if you had a manual trim and engine set at 90 deg and water flow skimming just under WDP you could make close tight turns on a dime. Been rigging lately Tohatsus 70 HP on hard hull boats, and that's same as rigging smaller engines, What liked about them, all were tiller steering converted to remote and the plus was all had clamps.

Really it's not a matter of boat type, hull, it's all about correctly sitting any engine on transom to obtain the best water peformance. AGCB is running an old 25 HP, say old due to having just 2" from WDP to AV plate heigt. On newer engines have posted all the variables you can have at only 2 possible heights. Over or under WDP. Check this out.

P1230065.JPG

This picture is a prime example of near cast on stone AV plate talking, at which lower tail height do you think water flow is passing by while on plane at a straight water course ? Anyone feel free to comment.

Happy Boating
 

Texasmark

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Dec 20, 2005
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I wonder how to measure depth while underway. There is only about 2" between def plate and AV plate.

Motor is late 90s mercury 25 HP and boat is Alumicraft V14. Trim is manually set w/ pin and I don't notice splashing over transom except a little from transducer. No easy way to raise or lower motor. May be stuck w/ it as it is.

Thanks.
Aaron
I had 12' and friend had a 14' Alumacraft back in the 80's time frame. Nice boats, flat bottom, keel of an inch or two, soft chine. We had no problems with the transom height on those boats. Both were 15" advertised. Engine power was 33 on the 14' with a side console. Never got the ruler and actually counted the inches of transom height. If you had a 20" engine on that boat with a 15" transom YOU WOULD KNOW IT! A lot of boats back then that were advertised as 15" were more like 13 and the AV plate was well below the water stream coming out from under the boat when at WOT.

As I mentioned earlier, get on calm water at WOT with your tilt pin in the slot you usually use and look over the transom at the water coming out from under the hull as it crosses the engine lower unit. Where is it with respect to the WDP and AVP?

Sea Rider,

You are running a different kind of boat than the norm around here. You have like 30 degrees of dead rise at the transom and around here only the deepest of the deep Vs are even approaching that much. Most that have dead rise it's more like teens of degrees and lots of boats, like his has none; flat bottom, rounded chine, slight V in the foremost 2' of the hull on a 14' boat. I can see where you have a different setup requirement than we do. Running that engine that low just amounts to a lot of unnecessary drag and loss of mph on boats around here. However, it does have the prop low enough in the water to seldom if ever blow out in a turn, and the prop is low enough in the water to all but eliminate surface detected prop wash. With these boats, how much bow wake you have determines how you did on your setup, course that is very hull dependent, and the softest riding deep Vs can't get rid of most of it since they ride in the water and not on it like a semi-v.
 

Sea Rider

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Texasmark.

Have installed engines on near flat, small, medium and deep V hulls following same exact procedure. The transom angle has nothing to do, that's what trim holes or power trim is all about, that is compensates the angle at which the engine sits on transom. Engine needs to sit perpendicular for hull to ride parallel to water level, provided that boat has deck weight even balanced. How can you suggest that boater needs to put his tilt pin in the hole he usually uses, that contradicts what is stated on ABC combo installations. With respect to WDP and AV plate heights will add 4 new variables that will turn you nuts if experiencing water splashes out/over transom and prop aeration.

1-Old boat model with old engine model,
2-Old boat model with new engine model.
3-New boat model with old engine model.
4-New boat model with new engine model.

Have read all engine Owner?s Manual brands, none of them states that AV plate must be even with boat's bottom, all states heights from 10 to 35 mm and that varies from brand to brand. My boat has nothing to do with his, both are different animals. Check attached link at post 16, that's same boat type, right. Determine if need a transom height optimization and go from there. If its a piece of cake or hard to correct having just 2" between both plates, it`s a different story.

Wanted to end saying that sitting an engine on transom and going by "the AV plate rule" says nothing. Once combo is placed on water and hull rises from water at speed, theoretical parameters will change and some dramatically. Check examples.

1-Sat a long shaft engine to a long shaft transom, as sen AV plate is 2" under boat's bottom.

P1230065.JPG

2-Went for a wot spin lightly loaded, well trimmed and water flow was passing right where red line is. Good performance at flat calm straight water courses, bit of prop aeration at choppy, wavy, windy sea cond and huge aeration at close tight turns at speed on flat calm water as well.

P1230066.JPG

Guess what, ended chopping transom down 1.5", now have exceptional water performance at any sea cond and close tight turns on a dime. And my engine is wot revving at 5750 rpm from max 5800 factory range.

P1230101.JPG

Happy Boating
 

Texasmark

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Messages
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[/QUOTE]How can you suggest that boater needs to put his tilt pin in the hole he usually uses, that contradicts what is stated on ABC combo installations.[/QUOTE]

Reason was to get him back to where he was when he had the problem. Also, other than splashing, he apparently already had the "sweet spot" for trim angle. Your last picture is what I wanted him to do and tell us what he saw. On paralleling lower unit to water flow vs boat wetted area and top speed, go to a Ranger or Triton, or Skeeter www and look at the pictures of their bass boats. Look at the angle of the boat vs wetted area, the engine trim position, and the flatness of the wake. Also if you are sitting in the boat you can hear 2 sounds: The whine of the engine and the hum of the prop blades hitting the surface of the water since they are partially out of it. These boats come with trim and jackplates whereby the engine can be tuned remotely for the water conditions, be it thrust angle, or depth of prop in the water. Obviously when the going is rough, you want your setup different and top speed is not your concern.


[/QUOTE]With respect to WDP and AV plate heights will add 4 new variables that will turn you nuts if experiencing water splashes out/over transom and prop aeration.

1-Old boat model with old engine model,
2-Old boat model with new engine model.
3-New boat model with old engine model.
4-New boat model with new engine model.

Have read all engine Owner?s Manual brands, none of them states that AV plate must be even with boat's bottom, all states heights from 10 to 35 mm and that varies from brand to brand. My boat has nothing to do with his, both are different animals. Check attached link at post 16, that's same boat type, right. Determine if need a transom height optimization and go from there. If its a piece of cake or hard to correct having just 2" between both plates, it`s a different story.[/QUOTE]

Again I think we are talking about two different situations and setups accordingly. I pulled up your Ballistic Rib boat to see just what it is. Quite a big water hull. Also noticed the comment about all the agencies who use that type of hull in their day to day workings, USCG, harbor police, fire fighting, etc.

Wanted to end saying that sitting an engine on transom and going by "the AV plate rule" says nothing. Once combo is placed on water and hull rises from water at speed, theoretical parameters will change and some dramatically. Check examples.

1-Sat a long shaft engine to a long shaft transom, as sen AV plate is 2" under boat's bottom.



2-Went for a wot spin lightly loaded, well trimmed and water flow was passing right where red line is. Good performance at flat calm straight water courses, bit of prop aeration at choppy, wavy, windy sea cond and huge aeration at close tight turns at speed on flat calm water as well.



Guess what, ended chopping transom down 1.5", now have exceptional water performance at any sea cond and close tight turns on a dime. And my engine is wot revving at 5750 rpm from max 5800 factory range.



Happy Boating[/QUOTE]

Well sir, your pictures are of a RIB hull and you are out for prop to water contact as a goal, not seeing how fast you can go in calm water; you just stated that fact. Apples and oranges.

Good boating to you too.

Mark
 
Last edited:

Sea Rider

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Some clarifications to get a properly psyched thought. We're talking about small to medium size boats, say in 12- 17 ft range & manual trim.

You can't achieve superb close tight turns prior matching sweet engine/transom height as prop will aerate if engine is not correctly seated. The sweet trim angle is the one that makes the boat to ride parallel to water level once on plane, provided that its well deck balanced. Any badly height seated engine will achieve that cond, but will inevitably aerate at close tight turns.

Sweet trim angle is not my invention, is stated on every engine brand Owner's Manual, the issue with most boaters is that they "don't read" OM, besides the issue of spot on engine height match is not specified in nowhere land. Had to dial it under experimentation myself due to being a maniac boater along running a small business selling boats and engines and can't possible sell a combo with performance issues, bad reputation for myself and business.

When you have finally dialed the sweet engine/transom height you will achieve :
-Best overall water performance on any water cond.
-Impeccable close tight turns at speed, will end up quite dizzy and probably vomiting..

Close Turns.JPG

-Faster hole shot.
-Max engine HP output and prop thrust while employing under powered engines on heavier applications.
-Better fuel economy at reduced throttle to maintain good plane.
-Perfect if afterwords going for a prop maximization to suit your boating needs up to a certain HP/Size point.

​Can't possible compare my under powered 18 HP Rib toy with those huge professional Ribs powered by 500-700-1400 HP engines, to each his own fruit. The sweet engine/transom height dials the best water bed amount for prop to grip as it should provided that prop faces water flow perpendicular for blade angle of attack to work gripping efficiently with least prop slip. Only achievable under sweet trim angle territory. As everyone loves AV plate realm, check what goes on while lower leg is ridding submerged inside water flow at speed by just varying trim angle off 0 deg. Did you know that sweet water is 1K times denser that air, now imagine salt water flow compacting itself along increasing drag on AV plate while lower tail rides through at speed at different trim angles.

AVP Trim Angle Effects.JPG

Chech WDP spot on territory behavior comparison.

WDP Trim Angle Effects.JPG

As can't possible have best of both worlds at same time, that's excellent hole shot and best top speed, went for one less pitch size from factory delivered prop which gives outstanding hole shot with slight less top speed, as opposed to crappy hole shot and slight more top speed. That Rib is rated for a max 50 HP engine, which don't intend to install as my current modest 18 HP takes me and brings me back from far away distances with no shock whatsoever. It's not how fast you want to get there, its all about how much you want to fully enjoy the water ride. 18 HP engines has been powering very efficiently my different 3.0 to 4.50 Mt boats models by just dialing sweet engine height, trim and deck balance.

Sorry folks for hijacking someone else's post, but boating theory is not plain compatible with real boating world well proven & documented practice. Lets hope AGCB resolves his boating issues and starts to have more boating fun than before.

Happy Boating
 
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