New 6 meter sib and problems

[spainman]

Hello to all,

I bought 1 month a go a new 6 meter sib and a new Parson 60 Hp Efi outbord.

Since I have launght it in the water I have been unhappy.

Maximum 21 knots, 4900 PRM and if I give quick full throttle, the RPM goes up but the boat do not go forward.

If I make it slowly it works more ore less.

I bought another prop to get more Rpm it do not work.

I am talking to the dealer and he told me the motor is new and Ok.

I do not know what the problem could be, The only I have seen is that the motor is mounted low, I mean the Cavitationplate is about 5-7 cm down the bottom of the hull.

The people arround told me that this is not a big problem, and do not have to cause these strange RPM problems.

Can somebody give some recomendations or help ?

Regards,
Arthur

[jeffstevens763@g]

What prop have you got on the engine

[Dankoozy]

Wow 6m for a sib is massive. what kind is it?

[Pikey Dave]

Originally Posted by Dankoozy

Wow 6m for a sib is massive. what kind is it?

My thoughts too, and 60hp is going to struggle.

[treerat]

Big boat, little engine. I'd say its too small for the load, guessing its not just you on board?

[jeffstevens763@g]

60 hp should be enough set up properly to be fair I would expect only about 22-23 knots max needs to get the rpm up though, for info I ran a 5m sib with a 40 hp and at work we ran an 11 m sib with twin 40 hp. Excel do a 7m 50 hp rated.


https://www.excel-inflatables.co.uk/...oat-xhd705.php

[spainman]

I send a photo of my sib.
It is an chinese modell but of a goog builder and with mehler pvc.

The 60 hp is ok it take maximum 90 hp

I tested it alone and with one person more.

What could be the problem ?

[TmMorris]

It may simply be the drag of the hull. You have a very large wetted area to force through the water and this is going to be amplified by the tubes also. Even on the plane the tubes are still going to be in the water creating increasing levels of drag which the motor must over come.

If you find a diagram of the torque curve for your engine you will see that peak torque will be below peak revs, probably by quite some way. The engine just won't have the power to overcome the drag because a SIB doesn't really get up onto the plane like a RIB. A RIB will rise out of the water allowing the tubes to clear the water and so while the outboard is running out of power the significant reduction in drag from the hull is a greater gain meaning that more revs can deliver more speed.

In the case of our SIBs the nature of their design is that that simply have a lower max hull speed than an equivalent RIb regardless of how much power we add. And I imagine that the longer the SIB the greater the drag is so despite being able to add a heavier engine to the rear it's extra power isn't diffident to overcome the extra drag.

I imagine that on a 6m rib a 60hp would give top speeds between 25-30 so hitting 20 with a flexible hull that's just skimming while dragging its tubes seems pretty good. Chances are you could fit a 90hp and se no real increase in top speed.

In short, there may be nothing at all wrong with your set up and it's just happily obeying the laws of physics?

[spainman]

Thank you for your good answer.

I have the following prop: original: 11 1/2- 13
other I bought to test: 11 3/4 -12

My problem is the following:

The first 10-15 minutes I reach this 22 knots with GPS, but after giving full throttle it goes down.

Up to 3.000 rpm giving slowly throttle their is an forward going, after RPM goes up and no forward.

This happens with both props.

Their must be something wrong.

Now I can just giving slowly throttle up to 3.000 rpm and after RPM goes up and the boat go slowlier.

The mecanic will come next week to make a testdrive.

I had some sibs before ( smaller ) and newe has this problem.

Zodiac and some other Manufactures make SIb´s with 6,7,8 meters.

[TmMorris]

It's possible that beyond a certain point the extra power is just causing hull deformation so increasing drag?

If the engine sounds like it's doing what it should is it worth considering the boat design?

Have you played around with the tilt of the engine? Does bringing it up a couple of notches change anything?

[Fenlander]

>>>It's possible that beyond a certain point the extra power is just causing hull deformation so increasing drag?

I have no experience of a SIB this size but I wondered that. From videos I've seen of 6m SIBs it does look as if they can be forced into a banana shape by lots of power and/or bend with the waves.

[909]

Could be losing pressure, either in the tubes or the keel. Could also be taking in water.

This creates more drag. That's why you start strong, and slow to a crawl.

Could also be your Chinese boat. I have one too, not a fan of them.

[jeffstevens763@g]

Tube pressure is critcle with a large SIB because it deforms if low on pressure.

You’re motor has a full throttle range of 5000/6000 rpm the correct prop would give you more top end assuming tube pressure and engine set up are all good.

I notice you have only had the engine a month has it been run in yet ?

[TmMorris]

The engine does seem, by all accounts, to be working as intended. It powers the boat into the plane and revs up.

There are probably far more variables with the boat than the engine that could be the cause of not exceeding 20mph.

Would it be expected that a SIB of this size could generally significantly exceed 20mph? To me that seems pretty good for something with so much wetted area and so much drag. The extra power of the engine is surely really there to ensure it can plane with the type of load a 6m sib would be likely to be carrying as opposed to delivering a high top speed?

The one area that I wasn't sure of is that there was a description of the engine revs increasing but speed being constant. Wouldn't that imply excessive prop slip? Once prop pitch, depth of prop and angle have been eliminated doesn't that pretty much mean it's hull?

[jeffstevens763@g]

For me the two props are virtually the same so not noticeable difference he needs to get the rpm up only at 4900 with these props and needs to come down on pitch or diameter to get nearer the 6000 rpm mark

[chipko]

Sorry to hear your problems, I too would be very disappointed with only 21 knots from 60hp.

If altering trim settings makes no difference and, as you say AV plate is 50mm lower than keel then perhaps it may well be the hull causing this ventilation.

Although I have no experience with sibs of this size some 3.8/4m hulls, air floor models in particular, can be prone to deformation at planing speeds. This can result in what is effectively a large air bubble travelling under the hull, exiting under the transom and causing the prop to ventilate…lose grip and over rev with no increase in speed. Can be similar to a hooked hull on a hard boat but perhaps without any noticeable porpoising.

Most common fix, at least on smaller sibs, is ensure everything (tubes, keel, floor) are at maximum pressure, distribute kit/fuel/passengers etc. and perhaps run with motor slightly more trimmed in than usual. Often this also seems to be more pronounced on very flat water.

Hope you can get this sorted.

[Jon Tallis]

Spainman....
When I was at University we ran a few 16ft inflatables with 40hp Suzuki's that definitely managed more than 20kts. Acceleration was slow but they got there. As I recall you had to feed the power on very gradually to avoid cavitation.
If as you describe, you are getting to a point where revs increase but speed doesn't, you are DEFINITELY suffering either ventilation or cavitation. A few others have mentioned the "air bubble" phenomenon with suggested fixes.
Having an observer look carefully at the engine leg as you accelerate may help confirm if you are suffering ventilation, either due to "air bubble" or due to air being drawn down the leg.
Your engine would seem to be set deeper than optimal for top speed but being set deep will help avoid ventilation so I'd advise not raising the engine at this stage.
If you are suffering cavitation, a larger diameter prop may help if you can fit one (see if you can borrow one), or you may just have to drive around the problem by feeding on the power very gradually.
Others have made fully valid points about tube pressure, weight distribution, engine trim also...

Good luck

[chipko]

Originally Posted by Jon Tallis

If you are suffering cavitation,

Oooh! I’d just love a motor with sooo much power it’ll literally boil the water around the prop. Nice. [emoji41]

[spainman]

Thank you to all the answers.

I will try this week maximum tube pressure and also keelpresure, and with the mecanic who fit the motor to get a solution.

Hopefully it will work.

[909]

You should be able to stand on your tubes with very little give.