Why I use a long shaft on a SIB

[Lee1]

I now only use long shaft outboards on my SIB's I know the general rule is to have the cavitation plate in line with the bottom of the boat but if your using bigger outboards engines (or over powering) your prop on a short shaft can spend a lot of the time out of the water even when planning on a smooth lake and even more so if your wave jumping. My personal experience of cavitation in both situations assures me that I'm better off using a long shaft, having looked at pics of thunder cats (who appear to be running long shafts) I'm even more convinced that if your running a big engine for power and speed especially in the chop it's a long shaft over a short shaft!

[beamishken]

Think you'll find zapcats & thundercats run short shafts and also run them high with surface piercing props or props suited to running high.
Running too deep your only slowing the boat down by creating more drag.
The theory in going fast is get as little of the prop in the water as possible whilst maintaining purchase on the water

[Fenlander]

Sorry Lee you're wrong for pretty well every SIB user and every SIB type.

From my experience with a couple of small hard hull planning boats, eight very different SIBs and over ten differing outboards a short shaft OB has been the ideal match to a short shaft transom.

In fact with our Aerotecs even with short shafts we often have to raise the OB further to stop excess splash being created by too much outboard leg in the water. It is quite possible to get a good balance between reducing splash and avoiding the ventilation you are worried about.

If you are suffering severe ventilation or cavitation problems with a short shaft OB on a SIB designed for a short shaft then if a hard floor type it's usually an inflation issue and with an air floor similarly an inflation issue or just a design limitation of the air floor at speed.

[jeffstevens763@g]

Don't you find the boat heels a lot and pivots on the engine making steering very light and twitchy from your top pic not much boat on the water?

[Mikefule]

The prop doesn't just push the boat forwards. There is a sideways component too, and if you have a long leg on the outboard, it will have more leverage, tipping the boat more. You might then try to compensate for this by positioning the engine asymmetrically (i.e. slightly to one side) and then you will introduce new problems because the forward thrust will be off centre.

Then when you are at anything below top speed, the longer leg of the engine will create more drag, sacrificing speed/using more fuel and making more splash.

Then when you come into shallow water you increase the risk of a prop strike on the bottom.

Or, to put it another way, if a long shaft really worked better, the manufacturers would have recommended a long shaft.

And finally - and here I write as someone who spent 35 years dealing with insurance claims - do you really want to explain to your insurers that your boat flipped or crashed when you were running an engine that breaches the manufacturers' safety instructions? Policies require the Policyholder to take all reasonable precautions to reduce or remove the risk of a claim arising.

[Lee1]

Originally Posted by Mikefule

The prop doesn't just push the boat forwards. There is a sideways component too, and if you have a long leg on the outboard, it will have more leverage, tipping the boat more. You might then try to compensate for this by positioning the engine asymmetrically (i.e. slightly to one side) and then you will introduce new problems because the forward thrust will be off centre.



Then when you are at anything below top speed, the longer leg of the engine will create more drag, sacrificing speed/using more fuel and making more splash.



Then when you come into shallow water you increase the risk of a prop strike on the bottom.



Or, to put it another way, if a long shaft really worked better, the manufacturers would have recommended a long shaft.



And finally - and here I write as someone who spent 35 years dealing with insurance claims - do you really want to explain to your insurers that your boat flipped or crashed when you were running an engine that breaches the manufacturers' safety instructions? Policies require the Policyholder to take all reasonable precautions to reduce or remove the risk of a claim arising.

My op was regarding running a large or overpowered outboard on a sib. The Police, insurance, running shallow and the river bank was not my issue, my post was how I keep the prop in the water planning or wave jumping at 30+knots by having the extra 5 inches length of a long shaft over a short shaft.
I posted a thundercat pic showing a long shaft leg outboard how the outboard is mounted and what prop is used is not the question or debate, the fact is the leg was a long shaft!! The cavitation plate was NOT in line with bottom of the boat. I've view other pics of thundercats out the water and they all appear to have long shaft engines, I know I'm not using a thundercat but I do have my stern out the water often in the swell of waves, You can't find any pics of SIBs wave jumping because there is no pics as such but that doesn't mean SIB users don't power boat like the thundercats.

[Lee1]

Originally Posted by beamishken

Think you'll find zapcats & thundercats run short shafts and also run them high with surface piercing props or props suited to running high.
Running too deep your only slowing the boat down by creating more drag.
The theory in going fast is get as little of the prop in the water as possible whilst maintaining purchase on the water

Defiantly a long shaft on that thunder cat, as in it's not a short shaft leg and the cavitation plate is not in line with the boat bottom. So I would say for SIB power boaters who want to thundercat go LONG


ATTACH]123730[/ATTACH]

[willk]

Originally Posted by Lee1

...having looked at pics of thunder cats (who appear to be running long shafts)

Originally Posted by Lee1

I posted a thundercat pic showing a long shaft leg outboard how the outboard is mounted and what prop is used is not the question or debate, the fact is the leg was a long shaft!! The cavitation plate was NOT in line with bottom of the boat.

I'm guessing that you thought this over, came to some conclusions and decided to use RIBnet as a sounding board? I get that you don't like the response. See how "appear" became "fact"? However, it does no harm to listen to the answers when you ask a question - especially when you ask the world a question - it has a lot of experience and cares not about what you believe. My (googled) tuppence worth as follows:

1. Ceasar Inflatable Boats, US - Boat models and information

2. Thundercat hull is a catamaran - different rules to a monohull.

Think about it.

[beamishken]

Originally Posted by Lee1

Defiantly a long shaft on that thunder cat, as in it's not a short shaft leg and the cavitation plate is not in line with the boat bottom. So I would say for SIB power boaters who want to thundercat go LONG


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Thundercats definately run short shafts check the specs, they look low because it's a cat so hangs below the hull to reach the water when planing

http://www.thundercatracing.com/about

[jeffstevens763@g]

BK is right just the angle of the shot makes it look long, i think you need to look at the relationship between tube diameter/depth and shaft length as with the futura design and bigger tubed sibs. it sounds to me you are trying to justify the long shaft for some reason but the overpowering and wave jumping clearing the boat from the water will only have one result i fear OMO

Ron Hale Marine Ltd - Zodiac Mark 3 Futura Aluminium

[Fenlander]

>>>Thundercats definately run short shafts check the specs

Yep my thought while I chomped my toast this morning. Folks tend to take dramatic pics of them catching air and it's quite hard to find one running fairly normally to demonstrate how the design works Lee. But the one I've attached below shows fairly well that in normal running their central "deck" the transom is on plays no part in contacting the water hence the cav plate relationship to the deck is nothing to do with getting the prop in the right place or best shaft length.

>>> SIB power boaters who want to thundercat

You can't Thundercat a SIB... different beasts completely.

[Roflhat]

Definitely short shafts on thundercats...
If you try a long shaft you either have the engine 5" too deep in the water and the boat will handle terribly and be slow with too much drag, or if you run the engine 5" up on the transom you'll have an excessively high centre of gravity and potentially roll the boat during high speed turns.

The cavitation plate is roughly in line with the bottom of the hijackers on a thundercat

[Xk59D]

I tried my long shaft on a suzimar sib and I thought the transom was going to fold into the water if you went near the throttle. I figured it was due to the engine but maybe wasn't enough pressure??....too late to find out as the sib went ages ago.

[HDAV]

Originally Posted by Lee1

, my post was how I keep the prop in the water planning or wave jumping at 30+knots by having the extra 5 inches length of a long shaft over a short shaft.

I’m sure you appreciate/the extra 5” but I think you kind of missed the point of wave jumping there..... drop 300 horses in a 3m hull and a negative shaft length and try wave jumping pushing 50knts and you’ll know what I mean......

Having fun and going fast aren’t the same, maximum thrust & minimal drag doesn’t make for exciting pictures.....

[9D280]

Originally Posted by HDAV

I’m sure you appreciate/the extra 5” but I think you kind of missed the point of wave jumping there..... drop 300 horses in a 3m hull and a negative shaft length and try wave jumping pushing 50knts and you’ll know what I mean......

Having fun and going fast aren’t the same, maximum thrust & minimal drag doesn’t make for exciting pictures.....

I assume you have tried a sharp turn with a longshaft engine on a shortshaft transom?

Aa while ago i dropped my rib aux onto a borrowed dinghy... it was either that or stay ashore. Aux is longshaft, dinghy was short. Even with only 5hp on a 3.1m sib and being carefully experimental I nearly had it upside down numerous times on the corners...

And the drag was horrendous - the engine was labouring the whole time.

But if you have a grunty engine & want to play in the waves go for it!

[Brage]

I’ve tried too long shaft on my sib (Zodiac FC 470). The engine was tuned Yamaha giving 40-50 HP. There was no major benefit with a too long shaft IMO. Even at top speed and rough waves.

The major disadvantage was the splashing into the sib and higher fuel consumption. The splashing was hopless when travling long distanse.

Cheers