Mercury Props.-

[JimS123]

Nope, in this precise example if the AV plate rides above the water level which is ridiculous or under what's the use of the doel fin, that I know a doel fin must ride fully submerged and parallel to the water level to work right, unless you want to use it as a step lader when climbing on board from behind...

Happy Boating

If a Doel Fin is submerged on plane it will cause drag and reduce your speed. On some hulls it could also make the boat squirrely. It needs to be submerged at idle to provide the needed lift to lower planing speed and reduce bow rise. But then at plane it needs to be just above and skimming the water level. With those 2 criteria met, you'll have an optimum setup.

Now, change props to get WOT right at max spec rpm and you'll have the best of both worlds.

Regardless, based on the picture provided in one of the early posts, that ain't no Doel Fin. I can't read the label, but my guess is that the "fin" is an SE Sport, which is not noted for performance.

Lose the fin, raise the motor so the AV plate is nearer to the boat bottom, dial in the rpm and then put on a real Doel Fin.

 

[QBhoy]

If a Doel Fin is submerged on plane it will cause drag and reduce your speed. On some hulls it could also make the boat squirrely. It needs to be submerged at idle to provide the needed lift to lower planing speed and reduce bow rise. But then at plane it needs to be just above and skimming the water level. With those 2 criteria met, you'll have an optimum setup.

Now, change props to get WOT right at max spec rpm and you'll have the best of both worlds.

Regardless, based on the picture provided in one of the early posts, that ain't no Doel Fin. I can't read the label, but my guess is that the "fin" is an SE Sport, which is not noted for performance.

Lose the fin, raise the motor so the AV plate is nearer to the boat bottom, dial in the rpm and then put on a real Doel Fin.

Spot on !

I’d want that AV plate running around an inch above the lower hull line. Or at least level with it.

 

[QBhoy]

Searider look at the two photos you have of the ribs in the air. Look at the height set up on the bottom picture. That’s the way it should be set up. The upper picture shows a set up that’s too low.

 

[Sea Rider]

Won't argue with you guys as this best lower leg height discussion will go on and on per secula seculorum. Don't see anyone actually owning a large Rib which water behaves differently to any other traditional boat.

Sorry guys a doel fin must ride submerged at all times once the combo is on plane, try this drag experiment on your own.

Run a car at the speed of 25 MPH, open the window pull your hand further out, place it parallel to the road, triim hand up and down, check the extra drag achieved at both scenarios compared to your hand resting at neutral position.

Sea hardened water at speed achieves 1 K more water drag than air. So if the doel fin does not ride parallel to the water level will achieve zillion times more water drag on the fin than the air test. Increasing speed will increase the drag. Probably not notorious on larger motors with higher HP ones. Personally dislike doel fins, are huge garbage collectors, have a set mostly used as paper weights.

QBhoy, you can't possibly tell by looking at 2 simple pictures if both ribs have their motors well height seated, happens that the lower one which is mine has 4 1/2 inch height difference between both plates while the first one has slight more than 2.0 inches compared. I'm plainly in agrement that the Merc installation parameters which are 4 works OK with the Bigfoot, but won't work same on the other motor as have tested it matching the 4 Merc lower leg height parameters and by water experimentation know what each height achieves in Our Water cond that might differ from yours. Each boat has their own motor installation which will vary beween each brand and model no matter if being same size.

What puzzles me is that No one has posted if the AV plate low setting you guys like going for works spot on in all water cond and at close u turns at speed or you guys happen to go boating on flat calm water cond at speed at straight water courses ?

Don't have it clear what you guys call Performance, for me it's all about any boat correctly propped that runs to it's full wot rpm range factory sated which it's the max that motor will go. If counting with a fixed load much better. I like a faster hole shot while losing slight top end speed which I never run, as there's too much fuel comsumption to achive at the most a lousy 1-2 MPH speed increase.

Without the need for raising the motor one hole up and by just correcting the motor's throttle cable response that only allows 80-85% of the 100% aceleration will achieve a notorious rpm increase difference. Hope the motor ups towards its min 5500 wot rpm range only checkable with another wot water test. If still around 5500 wot revs, will install a same style 10 pitch prop to bump wot revs towards it's max 6 K wot rpm range.

The water test will have to wait as the whole country is undergoing a 15 day forced quarantine due to the Coronavirus issue starting today. In the meantime I'm having a great water time in my bathtub LOL!!

Happy Boating

 

[JimS123]

Don't see anyone actually owning a large Rib which water behaves differently to any other traditional boat.



Sea hardened water at speed achieves 1 K more water drag than air. So if the doel fin does not ride parallel to the water level will achieve zillion times more water drag on the fin than the air test. Increasing speed will increase the drag. Probably not notorious on larger motors with higher HP ones.



What puzzles me is that No one has posted if the AV plate low setting you guys like going for works spot on in all water cond and at close u turns at speed or you guys happen to go boating on flat calm water cond at speed at straight water courses ?



Don't have it clear what you guys call Performance,

You probably got that right. Just don't see the point. Doesn't look like any fun.

I don't get the hand out window analogy. Obviously, the need for parallel goes without saying. But if you are saying that the Doel Fin above water will collect air, that don't make sense.

If my memory serves me correctly, I believe my first experimenting with AV height was about 1976. That was the year I bought a new motor with an adjustable transom bracket. Being a scientist I just had to run her up and down. But that was a long time ago. I thought these days everybody knew the plate had to be up high. (I sometimes get flat water, but usually its a 2 foot chop)

Performance means hole shot, top end, mid range fuel economy and all of the above.

Please stop saying "Doel Fin". Mister Doelsher's invention is the best of the bunch and his model never picks up trash.

 

[WesNewell]

SR, the water conditions don't really matter since where the water leaves the bottom of the hull right in front of the prop, the water is level without any chop. The only time there's any chop so to speak is when your whole boat comes out of the water. I've only done that 1 time doing ~60 mph and running across a cruisers wake. Boat engine and prop in the air. Almost lost it. Did lose one seat. 1983 15V kingfisher with 115 rude. I don't really care how you setup your boats. But please keep those erroneous setups to yourself. And second, if you setup your engine properly, you'll never need a fin of any kind.

 

[Sea Rider]

Answering your points Jim :

Haven said that a fin will collect air, what's the use of having one if riding highly exposed ? Asside from climbing from behind while used as a step ladder, the sole invention and intention is a faster hole shot and plane. My combo and all my friend's combos doesn't need a fin as knows how to distribute their deck load well, run fixed loads with their motors revving to their max wot rpm range factory stated. Our combos literally jumps on plane at 1/2 to 3/4 throttle.

The window analogy is to show that if a fin does not ride parallel to water surface will achieve and increment the fin drag, much morei if the speed is highly increased, notorius fin drag with under powered motors. Will need to count with a installed tinduction tach and a GPS to check that out.

There's a big water performance difference sitting a motor on an adjustable bracket the ones that protrude out transon and a transom mount which raises the motor at same transom level. That's why I don't install any of those ir required, just go for a transom height optimization or a transom chop down when required as have the means and qualified fiberglass techie with 30 years of experience dealing with fiberglass. We don't have 2 foot chop, at the most 1/2 foot ones in exceptional odd days.

At least we are in plain agreement about what Performance means, you get max performance out of your motor while water flows by the AV plate, me where water flows skims right under the upper splash plate while our motors runs towards the max wot rpm range. With respect to Performance was answering to what was posted at the bottom of post 38 with which I don't agree at all.

Oh come on, all fins whichever brand they are will pick up floating trash, garbage, kelp, plastic bags, much less if no fin is installed. The wider the fin's horizontal surface the more garbage that will collect.

As this post has escalated away from what was inquiring in the original post will leave it there for the moment, after quarantine ends will post updates, stay tuned....

Happy Boating

 

[Sea Rider]

SR, the water conditions don't really matter since where the water leaves the bottom of the hull right in front of the prop, the water is level without any chop. I don't really care how you setup your boats. But please keep those erroneous setups to yourself. And second, if you setup your engine properly, you'll never need a fin of any kind.

Hey WN you turn yourself to a real champ when making idiotic comparisons between both HP motors. What about a prop revving high while working under chopp and close swells one after the other. A prop will aerate more under both water condidions if sitting too high while the one sitting much lower will experience the least or no prop aeration at all as the prop bits a higher amount of water.

Touche, would highly recommend you to keep your own erroneous set up extrictly to yoursefl, never said that my motor needs a fin or that I love them., should read well my posts, wild guessing is no bueno mate !!

Don't understabd at all why you get so darn pissed with my answers, that I remember well have not asked for your help or tech advise at any time on any post, so you better position yourself well...


Happy Boating

 

[Scott06]

Don't understabd at all why you get so darn pissed with my answers, that I remember well have not asked for your help or tech advise at any time on any post, so you better position yourself well...


Happy Boating [/QUOTE

sea rider kind of funny post considering you had to be educated as to what engine you were working on here? Seems like you were looking for answers now you’re POed at people ?

 

[Faztbullet]

 

[JimS123]

Clever advertising scheme by the Attwood Corporation. But in effect, its true. The setup with the "flat" foil is the reason why so many people claim loss in speed, poor handling, picking up garbage, etc.

Unfortunately, the package merely tells you how to drill the holes and mount the foil, not to optimize the powerplant with a foil on it.

Makes no matter to me - I'll just continue to enjoy the benefits.

 

[Sea Rider]

Used to go boating with a 15 HP motor with a Doel-Fin brand installed, at that time didn't play with an induction tach to check if the motor was running at least middle to max wot rpm range or monitoring the rpm reduction if any while running with or without the doel fin. The boat run with deck weight evenly distributed, the motor sitting at the sweet transom height with the fins fully submerged.

Ended uninstalling them, the boating beach was a natural kelp farm, ended overheated the motor badly twice due to lower water intakes blockage at speed, asside from collecting huge quantities of kelp on the fins.

You should always go for a prop maximization soon after the motor's break in period ends, when buying second hand combos, used motors, having hydrofoils instalations and prop guards. Can assure that the boaters who had high expectations instaling fins on lower leg did not take into account what I am saying mainly because not knowing anythig about it.

Sorry Scott but the poster's answers is not an educated one, happens that the Merc Installation Parameters contemplate 4 different height parameters, can use whichever one you like or works best, will depend entirely on your type of boating, installation prefferences, etc.

A Necessary Clarification : The purpose of this nice boating forum is to give, seek, share tech information, have a great time with other boaters, share boating experiences among others, but if you keep receiving the same cast on stone answer from the same poster everytime the lower leg height is mentioned in my posts including being extremely impolite the tone of my response to that particular boater will change dramatically. Let's not lose the cordial online nettiquette...

Happy Boating

 

[JimS123]

A Necessary Clarification : The purpose of this nice boating forum is to give, seek, share tech information, have a great time with other boaters, share boating experiences among others, but if you keep receiving the same cast on stone answer from the same poster everytime the lower leg height is mentioned in my posts including being extremely impolite the tone of my response to that particular boater will change dramatically. Let's not lose the cordial online nettiquette...

Well said.

Also, .....when my opinion is refuted by several (all) others, it makes me go back and think maybe I need to re-evaluate my stand and maybe make some more experiments.

Or,....when I truly know that I'm right (after 50 years of experimentation), after being refuted several times I just let it go and chalk it up to youngins that don't really know.

Irregardless (TIC), we are here to learn, and to teach, and be amused by some of the comments.

 

[Sea Rider]

Responding to you post, this forum is like the Parliament, many will never be in accordance with your ideas. Didn't know you had many refuted posts being a long term experienced boater. If you have experienced to satiety and obtained the same results on a given tech post, there will always be boaters refuting them as many here won't be duplicating them to effectively check that this plainly agrees with your post.

Find newer boaters with 0 boating experience to be more open minded to follow what experienced boaters with well documented experience are suggesting, it's a matter to duplicate them on their own and boat happily soon after.

Happy Boating

 

[JimS123]

Use of the word "I" is sometimes done to be respectful and non confrontational. Often times use of the word "you" is offensive and merely puts the other person on the defensive.

These comments are meant to be conciliatory and it is assumed that the issue at hand has seen a successful resolution.