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Outboard shaft conversion


Darvster
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Jay Jay is now all running, re-wired completely, toys installed, locks everywhere and now on the mooring with a couple of trips out this week (without any disasters).  However, being my first boat and my very limited boat knowledge when I bought it in January, it seems that the long shaft outboard is incorrect for the type of boat.  

 

In a previous post, I mentioned that on my first trip out of Lymington back in March, I did not seem to be getting the required speed when coming back in against the tide.  I presumed that the problem was the prop size. Following Brian's advice to contact Prop Solutions with all the information, when out in the bay this week, I measured the speed, rpms against WOT etc etc.  My engine is a Honda 75hp Longshaft with a WOT rating of 6000 of which I was only achieving 4500 revs.  After speaking to Prop Solutions and before I had even completely given over all the information, they confirmed that the issue was with the shaft length and not the prop size.  Apparently for every inch the cavitation plate is below the bottom of the hull, drag is created and reduces the revs by approx 200-300 per inch below. 

 

Although this does not really create any real issues, apart I am not getting the performace I should be and when at speed, the bow rides very high.  This therefore gets to my question which is has anyone converted a long shaft outboard to a short shaft and if so, how difficult and costly is it?  

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Hi

 

No need for any major work, just lift the engine up to line up the cavitation plate and hull, this should have been done on first fit.

 

It is why an engine bracket has a selection of holes, just a nut and bolt job mate.

 

Phone me if you want to talk it through, phone Dan if you want someone to do it for you.

 

Charlie

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Thanks Charlie, that sounds simpler.  When I am next on the boat I will have a look to see exactly how far I need to raise it and also how much movement there is on the engine bracket.  I will let you know how it looks.

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Just had a look today and it seems that the engine has been fitted as high as it possibly could.  So, back to my original question regarding the shaft conversion or alternatively, is it easier to raise the rear transom somehow to allow the engine to be bolted in a higher position?  Any other suggestions?

 

2015-06-27%2015.00.47_zpsnkijjawa.jpg

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  • 1 month later...

Just an update on my engine issue and with some great help from Hooky, I think it has now been solved.  A few weeks back, Hooky managed to find a jacker plate on ebay which seemed to be the most cost effective solution.  So with today planned, the boat out of the water (amazed at how much algae had accumulated over the two months in the water), parked up at Hooky's we set about it.  Looking at it, it all seemed so simple, take the engine off, bolt the plate on and re-attached the engine.  Simple.  If only it ever worked out that way.  The jacker plate did actually fit fairly easily, it was removal and hoisting of the engine that took the time and in the end with some brute force we managed to get everything fixed back.  So the engine has now been raised some 4-5" and also, Jay Jay was treated to a new battery and a shiny new prop.  Unfortunately by the time we finished, the tides were not suitable to get it back on the mooring or even a test drive but many thanks to Hooky for all the help.  Now got to wait until next week to see how much of a difference it has made.

 

2015-08-15%2017.07.10_zpsmig93dm9.jpg

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  • 2 weeks later...

After having Jay Jay sat on a trailer next to the house for the last two weeks and having been itching to get her out but unable to do so due to work commitments, I eventually managed to get her back on the mooring Friday evening with a view to testing the new engine position.  

 

Saturday morning the weather looked good so with wifey in tow I took her out.  The first big noticeable difference was the control and responsiveness from the new prop whilst getting of the mooring although I must admit the last prop was pretty battered due to some rookie mistakes coming back through Christchurch run and harbour on some fairly low tides.

 

Once out of the run, I let it rip to see what would happened and what a difference.  Firstly the boat seemed to literally jump up on plane rather than the gradual increase in speed previously experienced.  With full throttle down, the revs were now achieving around 5500 of the 6000 WOT rating rather than the 4000-4500 revs I was managing to achieve.  Giving it all, much to the dismay of wifey, the boat got up to around 27 knots and it was not long before wifey got uncomfortable/nervous and I was sternly told to slow down.  I think that with a little longer run and some tweaking of the trim it could achieve 28-29 knots which is far more than the max 22 knots I was achieving.  In addition, the boat seemed to handle a lot better especially cutting through the waves and swells.  SHe now cruises between 19-20 knots rather than the previous 12-15 knots.

 

Overall well worth the effort of fitting the jacker plate. Next step is to remove the hydrofoils fixed to the cavitation plate and see what difference that makes.

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Must admit between the both of us raising the engine and getting it somewhere close to what 'might ' be correct is a bit of a gamble.

I initially thought that not only was the engine far to low it also seemed a possibility that due the deep V shape of the hull I noticed a large amount of turbulence going over gearbox and propellor , was this a route crux of the other problem ?

 

Problem B seemed to occur in a following sea, ' broaching' pushing the bow far more than seemed comfortable ; another guess ( from me ! ) was the dole fins ??

 

Blind leading the Blind !! The power - lift I got also had extensions that could also move the leg around ten inches further back from the transom , Lee took the decision to leave these off for the present, it could in theory create other issues , cables may not be long enough and for every inch you drop back you should also add another inch of lift !!!!!!! ????

 

The other thing to consider is the prop , the main and most important of all , we put on a brand new one in standard size, no information can be found on the net ( or anywhere else) as Jay-Jay is a very , very rare beast , a Texas 530 , I'm not positive but there may only be two in the UK ( Hull might have been copied by Quicksilver at some point )

 

So from Lees post it seems as if we may have been very lucky to get it spot on first time around , we shall find out more tomorrow with a full crew full of Pies etc ,

 

Fine tuning ? Probably , is so difficult to know what direction to take to find the best compromise between handling, performance and balance , especially when I'm far away from truly knowing what's happening and Lee is a expert in fitting a " totally awesome " sound system """" cool tunes and all !

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Dole fins are dangerous if you ask me, my boat had them when I bought it and they were great until there was a swell and trying to cut across it they would almost bite and make the boat pitch. Removed and replaced with smart tabs, much more stable and instil confidence!

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I think fins work only on a certain type of hull, on my Shetland they work fine, yes the loose a few knots top end but a definite positive on fuel consumption and yes , she gets up on the plane far quicker .

On my mates Wilson 23 I really think they seem to be negative on the handling ( tight turns) but again using them makes the ride far more stable in bumpy conditions .

Other thing noticed is weight distribution is critical , fins on again feels better against tide but absolutely crap on Jay-Jay in a following tide , funny thing is it makes no difference on my own boat ???? Weird , trail and error, .

 

If that's the case why do so many boats with outboards have neither trim tabs or fins and are just about perfect in ( nearly) all conditions ???? Maybe modern hulls are getting more efficient , . The most convincing thought is the manufactures spend millions of squid developing engines and not one of them has ever produced ' hydro fins' as standard fitment

 

I think from years of setting up bikes for racing one day is never the same as the next , what suspension setting that works on one circuit is useless on another , with water 8000 times more dence than air its makes it difficult if your not a expert .

 

I suppose it really is about a compromise , horses for courses . We shall see , early days yet .

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