AV Plate > Water Flow - Page 3

Max... · 12 June 2014, 18:15

Yep, cheers, will probably leave it as it is with height as per pic and standard prop - it picks up really well and 18 kts is fine really.

Not had my tiny tach on it yet to check WOT RPM but will soon.

Locozodiac · 13 June 2014, 01:02

Hey Max.

Forgot to ask you, told that on your small standard sib, dialing lower WDP against water flow (Sweet Height Spot) worked very good. Was wondering when outing by yourself on flat calm no wind water. If going full grip to plane faster the bow will rise accordingly, do you need to move yourself forward for bow to come faster down ? Or bow eventually will come down annyway but will take bit longer without in the need to move yourself forward. Which one applies or both does ?

Don't play with props or prop pitches by suggestion without a induction tach, very bad musique for your engine. That's what tachs are, to measure max wot rpm factory stated for that engine..

PD, still writting my assay as homework..

Happy Boating

Max... · 13 June 2014, 06:47

Loco, wasn't quite sure what you mean but my normal loading is me, one child and (like all SIBs they need some load up front to make them less 'bow happy') a Pelicase with anchor and gear midships. Boat pops up on the plane very nicely and bow drops quickly.

Locozodiac · 13 June 2014, 15:26

General question : Let's say that on a dry installation at your house, you have dialed a perfect AV plate even or slight lower than keel/boat's bottom.

-What do you expect your sib to achieve ?

-Did you take into account that water flow will pass under lower tail when sib is on plane at speed. Yes, No.

-Not even considered it ? Short answers please.

Max, was asking because on my previus 360-380 sibs with engine perfectly height seated didn't need move myself forwward or place weight up front, just went full throttle, bow whent happily up and in 3 seconds literally jumped on plane, that was with a 18 HP, both sibs rated for 20 HP.

Happy Boating

Projectile · 13 June 2014, 18:34

Quote:

Originally Posted by Max...

Goodness knows why Bombard don't change the transom shape with a shallower cut out on top so we dont't need to comprimise like this with raiser blocks, too costly for them I guess in tooling or whatever.

I completely agree with you. There's something wrong with this whole industry.

Imagine that when you bought a car, you had to buy tires and you had to trial different types and treads and widths to get you car to just run properly. Imagine that you had to modify the suspension just to get the car to run properly with a particular tire. I'm not even talking about milking every last drop of performance out of it - just to get it to run properly.

The transom length on my boat was 18 inches and the shaft length on my outboard was 22 inches. Why couldn't they both just be 20 inches?

I had to have my transom plated to raise the motor 2.5 inches. Without that major modification to my transom, the twisty clamps that fasten the motor to the transom would have been above the transom on 2,5 inch lifting blocks.

One should be able to get their boat to run well or at least reasonably well by just dropping an outboard on the transom and going.

SIBRider · 13 June 2014, 22:53

I suspect they have their reasons still somewhat puzzling.
My boat is under-powered for sure running 25 hp while rated for 40 but performs reasonably well.

Here it is under go at 22 knots with just me

Still planes perfectly well with 3 adults and 3 kids on board. Tops out at about 18-19 knots under that load.

Projectile · 14 June 2014, 16:24

FC470s are great boats. The futura hull really helps with getting the boat to perform better with less power. I love your boat. How did you find and get it?

Projectile · 14 June 2014, 16:25

Does anybody have personal experience with how much difference a stainless steel prop makes vs aluminum? I'm particularly interested in how much, if at all, it reduces cavitation on turns... and any other differences you think SS really makes.

Projectile · 14 June 2014, 16:26

Can anybody recommend a good iPhone app for measuring boat speed?

Locozodiac · 15 June 2014, 00:45

Quote:

Originally Posted by Projectile

The transom length on my boat was 18 inches and the shaft length on my outboard was 22 inches. Why couldn't they both just be 20 inches?

One should be able to get their boat to run well or at least reasonably well by just dropping an outboard on the transom and going.

Are you using a short or long shaft engine on a 18 inch transom ? If a short, its too short, probably water flow at fast speed/plane will pass skimming around AV plate, if so will have prop ventilation on choppy seas and at close turns.

As you have experienced, a boat/engine is not a car. It's not a matter of just droppimg engine on transom and voilá, in most cases will need to fine tune engine height for top performance. When sitting any engine need to have 3 important issues into : transom height, engine brand and very important tube's diameter.

That's why is irrelevant on sibs AV plate distance on a dry installation at home becuse you don't know how much or less sib will float once on water. As a enecdote, we installed 2 new 18 S Tohatsu on 2 diffrent lenght sibs. On the 380 experienced excesive water splash over transom, 420 worked perfect, why was that so ?

Very simple, 380 had 38 cm transom height and 45 diam tubes, 420 had 38.5 transom height and 50 diam tubes. The 420 floated much higher than the 380, had to raise 380 transom 2 cm to perfom exact same as the 420. The wider 420 diam tubes compensated having a shorter transom height compared to a higher one on the 380.

Happy Boating

Projectile · 15 June 2014, 03:17

Quote:

Originally Posted by Locozodiac

Are you using a short or long shaft engine on a 18 inch transom ? If a short, its too short, probably water flow at fast speed/plane will pass skimming around AV plate, if so will have prop ventilation on choppy seas and at close turns.

If a short shaft transom is supposed to accept a 15 inch shaft outboard, and a long shaft transom is supposed to accept a 20 inch shaft outboard, why would a SIB manufacturer make an 18 inch transom? And why would an outboard manufacturer make a 22 inch "long shaft"?

(The answer to your question is that I have a long shaft outboard that measured 22 inches.)

Quote:

Originally Posted by Locozodiac

That's why is irrelevant on sibs AV plate distance on a dry installation at home becuse you don't know how much or less sib will float once on water.

A) Right, I won't know that. But the manufacturer surely ought to know that and I see no reason why they don't take that into account when they determine the transom height. Do you?

B) The height of the AV plate relative to the transom bottom is not irrelevant because before you put the boat in the water, you have to place the engine at *some* height on the transom, so it makes sense to consistently consider a single motor feature (AV plate) and a single boat feature (transom bottom) and work from there. Would you start out by placing the AV plate 3 inches (7.5 cm) below the bottom of the transom? Plus, say that one finds an ideal height for the engine after testing, but the engine breaks down and needs to be replaced. How does one determine where to put the replacement? Wouldn't it make sense to at least aim for the same height as the first motor, and how would one know what that was if one didn't know the height of that motor relative to the transom bottom? Lastly, and perhaps most importantly, these are the 2 standard reference points that everybody in the boating world talks about when discussing motor height.

I wish that SIB manufacturers would at least test their boats the way that you do, and publish the range of motor heights (AV plate relative to transom bottom) that produce reasonable all-round performance (as you did in this thread) for the recommended hp motor of the leading manufacturers or even just one manufacturer so that the consumer could just buy that motor and be done with it. Evinrude and Zodiac are "preferred partners". Mercury manufactures inflatables so they could do that for their own boats. Etc.

Don't you think that's reasonable given the current state of affairs where they pass along a lot of guess work and refinement that is time consuming and labor intensive to the customer?

Projectile · 15 June 2014, 04:35

Oops. I mistyped: The transom was 19 vertical inches, not 18, as I wrote. So my AV plate was 3 inches below the bottom of the transom.

Locozodiac · 15 June 2014, 15:49

To find a perfect match would be mission impossible, keep in mind that all engine manufacturers don't makes engines for specific inflatable/ribs uses, are for overall marine uses. In most cases you have to adapt a given brand engine to a given brand sib if wanting boating perfection.

When didn't know a bit about AV plates, transom heights, my first OMC 310 sib was state of the art transom matched to a S EVI 9.9 engine, what was so ? Simple, both were OMC products. When tested a Tohatsu S 15 on same 310, all my splashing troubles began.

Definitely when you sit an engine on transom and boat is placed on water you have a starting water level pont, but that doesn't say much as you need to test trail that combo and fine tune transom height accordingly to match it to that particular engine brand, as you have personally experienced.

Happy Boating

Projectile · 15 June 2014, 17:01

Quote:

Originally Posted by Locozodiac

keep in mind that all engine manufacturers don't makes engines for specific inflatable/ribs uses, are for overall marine uses.

Loco, you are an expert when it comes to this topic, and I always appreciate your kind opinions, but you are one stubborn guy, too.

We are talking past each other, here. I'm saying that the industry should standardize, and you keep responding by saying that the industry is not standardized.

I see no reason why the motor manufacturers can't standardize the shaft length for outboards that attach with twisty clamps and the SIB manufacturers can't then design transom lengths accordingly so that the vast majority of people can get decent performance by just mounting the outboard on the transom and going. Saying that the outboard manufacturers make motors for a variety of uses is no reason why the shaft length can't be standardized across different manufacturers. After all, they all make outboards for a variety of uses.

Anyways, an industry that is standardized for the benefit of the consumer seems like it will remain my pipe dream.

Quote:

Originally Posted by Locozodiac

Definitely when you sit an engine on transom and boat is placed on water you have a starting water level pont, but that doesn't say much as you need to test trail that combo and fine tune transom height accordingly to match it to that particular engine brand, as you have personally experienced.

Agreed. But it still makes sense, as the motor height is adjusted in relation to the transom, to always talk about the height of the AV plate in relation to the bottom of the boat for all of the reasons I mentioned earlier. But I can see that I will not be able to change your mind about employing this standardized way of describing motor height. That's fine, of course. As long as at the end of the day, everybody can communicate effectively. I suspect that you will have an uphill battle, though, always having to translate your "water flow over AV plate" to "AV plate relative to transom bottom" in threads like this.

Happy Boating!

Locozodiac · 15 June 2014, 17:36

Short answer. If all engine manufacturer would built engines to match each particular brand boat's transom height to state of the art perfection do you know how many thousands of engines would be required to be built for that just insane and cracy purpose.

Definitely we don't agree so far about Deflector plate, AV plate in relation to a given transom height with respect to "standar inflatables", if you still want to go to your particular inflatable type side it's useless to keep maintaining this by now extreme controversial conversation, because both have different hull shapes, tubes, speed tubes, etc. To each his own, period.

If I were talking BS here, how come Max has achieved much better overall sib performance by just matching what's been stated on the boating guideliness. If worked for him why would it not work same for someone else ? The main issue is that most boaters are lazzy, gets tired just by thinking of all the extra work that must be done to achieve perfect boating.

BTW, like rowing agains the current...

Happy Boating

Projectile · 15 June 2014, 18:23

But I'm not suggesting that the engine manufacturers make motors to match certain SIBs at the exclusion of other boats. I agree with you that that would be ridiculous.

I'm suggesting that the motor manufacturers standardize the motor shaft lengths and that the SIB manufacturers then design boats with transoms with heights that provide reasonable performance for those industry standardized shaft lengths.

Please read my posts carefully and charitably: I never said that you were talking BS. In fact, I complimented you on your expertise.

I am not here to fight. I've shared my ideas, You've shared yours. I haven't insulted you at any point, though I have disagreed with the language that you use to describe the relationship between the motor and the transom. As long as we all understand each other at the end of the day, that doesn't matter.

Please keep up the good work.

SIBRider · 17 June 2014, 15:33

Deflector plate is irrelevant. Some engines have two some have one, located at different height etc. It should be above water. That's about sums it is up.
Relationship between A/V plate and transom bottom determines water flow around skeg, amount of drag, etc.
It doesn't matter how low boat/motor seats at stand still unless we talk about cat type boats. Single hull boat under go displaces water. Effective water-line at planing speed immediately aft transom is level with transom. Water starts rising immediately aft. Distance from skeg to transom determines how high water rises toward natural surrounding level. Same vertical height transom may have different flow around skeg depending on transom angle. More acute angle - further skeg is from transom (simple geometry). Higher motor can/must be raised for best performance. That's why they use stand-off brackets on high-speed boats to reduce drag and have prop grip less disturbed water.

There are only handful of outboard manufacturers in the world and hundreds of boat manufactures if not thousands. SIBs are barely a blip on their radar. They are fairly unified in shaft length 22 and 21 inches pretty much. It's up to boat manufactures to make sure fitment.

Clamp on outboard is a compromise. Compromises usually don't really perform absolute best. Zodiac work boat or Futura Commando are serious heavy duty boats unlike most other hobby craft. They are expected to perform in very bad conditions where splashes and/or top speed and economy are less of the concern. When you are in 4ft seas you will want to have you prop as low as reasonable to keep as much grip as possible having reasonably low transom gives that option. Ultimately our boats call for 40hp+ even 60hp+ motors which usually are bolt-on. Selection of bolt holes gives sufficient height adjustment to achieve good performance in most cases.

Projectile · 17 June 2014, 15:48

SIBRider: good points.

Max... · 17 June 2014, 16:04

I have to agree, very good post.

The 'standard' we have is about as standard as it will ever be or probably needs to be as there are so many variables.

Locozodiac · 17 June 2014, 19:51

Max, when standard sib and engine height were tunned on your combo you had excellent performance, right. Know you have passed to other side with your other sib and joinning Proyectile, Sibrider club on this usseless engine match crusade.

As said way before what stated on the ABC boating guideliness will NOT work same on your different inflatables, why do you continue with this nonsense post here. You should start your own post, post all your theories one after the other, go for a wot test, check, experiment, adjust, whatever needs to be done under trrial and error. Will know that theories specially yours don't work well on real wet boating world.

My essay should be ready in 2 more days, it's written as for a child to follow it. But again NOT INTENDED FOR YOU, but have a look anyway, it's just for standard sibs/ribs boatesr that wants to follow as it's not stated on any booklet, owner manuals.

Let's all hold our horses here, it's useless going 3 against 1 on subjects that don't correspond at all.

Happy Boating