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Old 27 October 2003, 10:58   #1
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Searider propeller ventilation / cavitation - how to prevent?

Have just had a new transom put into my 5.4m Searider. All looks good but I don't think that I can trim up as much as I could before.

When I trim the engine up all is well until I get a step increase in rpm of approximately 1000 from 4500 to 5500 with no increase in boat speed - ventilation or cavitation I guess.

This is preventing me from flying the boat properly - it now seems to plough through the tops of the waves more than it used to.

The engine is a 90hp mariner 2 stroke and I get about 5500rpm and about 39/40 knots.

I confess that I did not take any measurements of where on the transom the engine was prior to the surgery but assumed that it would go back in the same place.

The cavitation plate is level with the bottom of the boat and the engine is offset 1.5" to starboard. The offest is approximately the same as it was before.

I have spoken with Avon who told me the cavitation plate could go anywhere between 1" above to 1" below the bottom of the boat and that offset could be anything up to 2.5" or 3". So I guess the engine is approximately in the correct place.

I have adjustment in the mounting holes to lower the engine about3/4" but would need to trim the top of the transom to do so. This would be a bit of a work up as I don't have lifting gear and was charged over £200 to lift off, store and lift back on the engine when the transom was done.

Alternatively could a change of propeller make a difference. I have a standard aluminium propeller at the moment.

All opinions would be greatly received.

Also for my next boat what should I get?

1. 6.5m Porter/Coastline with a 120hp Mercruiser Diesel, or
2. 6.5m Scorpion with 150 Optimax/HPDI

Have driven the former and it didn't hit the right buttons but would be cheap to run.
Haven't had a try in a Scorpion yet but I think they look fantastic but I guess will cost (a bit) more to run.

Duncan
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Old 27 October 2003, 12:26   #2
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Hello,

On the transom note, I played around a bit with the height of engine on the back of my boat earlier this year to see if it was possible to gain an extra knot out of the top end speed..... in actual fact, it didn't gain me anything other than cavitation.

What I found was that when the cavitation plate was exactly level with the keel of the boat, you could feel the prop loosing bite on the water when putting large amounts of power on from a standing start. When up on the plane, I could not lift the engine above the mid-way trim point without getting serious cavitation which was very much counter productive on the speed side. I also found that in a bit of chop, the prop would loose bite, and I'd have to drop the revs down for it to regain proper propulsion once more.

As it is now, the cavitation plate is about 1cm below the keel of the boat, which seems to provide the best of all worlds for me. Apart from when trimmed right out, the boat will go into a hard corner quite happily and come out without any hint of loosing bite on the water. I also now have full range of trim, and when trimmed out can get the best speed on the boat.

Your best bet may be to lower the engine by one notch, and see if that makes any difference, either better or for worse - if it's worse, try lifting it to one notch above what it was before.
Unless anyone on the forum will say for definate if it's something else, it could just be a trial and error situation I'm afraid

-Alex
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Old 27 October 2003, 12:39   #3
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Re: Searider propeller ventilation / cavitation - how to prevent?

Alternatively could a change of propeller make a difference. I have a standard aluminium propeller at the moment.



Duncan [/B][/QUOTE]

try s/s lazer 2 prop
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Old 27 October 2003, 13:46   #4
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Re: Searider propeller ventilation / cavitation - how to prevent?

Quote:
Originally posted by Searider
Also for my next boat what should I get?

1. 6.5m Porter/Coastline with a 120hp Mercruiser Diesel, or
2. 6.5m Scorpion with 150 Optimax/HPDI
Do you need to ask?
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Old 27 October 2003, 13:58   #5
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Sea Rider, are you the guy I was talking to in Cowes about a month ago, I was in a blue Scorpion and we had our big Sea Rider out at the time.

It doesn't matter if you are or not really, but as you are local if you want to have a go on the Scorpion send us an email and we'll meet up. (it's not for sale though), but |I am still plesantly surprised by my fuel bill. I think we get about 35 LPH at 50 mph

Obviously the diference between a Coastline with a diesel and a Scorpion with an Opti is going to be vast. The Coatline will be cheap to run however and the people who own em seem to like em.

But if you have had a searider with a big engine then you obviously enjoy acceleration which you will certainly get with that package.

cheers
Stuart
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Old 28 October 2003, 03:04   #6
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Alex,

Thanks for that - I've often heard that raising the outboard a bit improves top speed but from your experience it would seem that my best bet is to lower mine.

Time to get out the jigsaw perhaps.

Regards

Duncan
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Old 28 October 2003, 03:09   #7
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Rockey,

Is a lazer II better at running super cavitating / in aerated water or is it more suitable for giving me bow lift.

I have heard of custom race props being able to be optimised to give either lift or sinkage at the stern but feel that they might be expensive - certainly more expensive than modifying my transom.

I've also seen "high 5" props on some fast ribs are these any good?


Does anyone have a lazer II suitable for my engine that I could try for a weekend prior to slicing my transom down?


Regards


Duncan
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Old 28 October 2003, 03:12   #8
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Richard,

I probably don't want to ask which boat next - My heart wants a Scorpion but I was giving the diesel boat owners the opportunity to tell me how good a 6.5m with a 1.7tdi can be and let my head buy a cheap to run diesel boat.

if a 6.5m with a 1.7tdi would do 40 knots I might be tempted.

Duncan
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Old 28 October 2003, 03:41   #9
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Quote:
Originally posted by Searider
Richard,

I probably don't want to ask which boat next - My heart wants a Scorpion but I was giving the diesel boat owners the opportunity to tell me how good a 6.5m with a 1.7tdi can be and let my head buy a cheap to run diesel boat.

if a 6.5m with a 1.7tdi would do 40 knots I might be tempted.

Duncan
Duncan

regret but no 6.5 nor 6.3 rib fitted with the Mercruiser 1.7 TDI will do 40 knots. The max you will get is 32 knots and that is pushing

Andre
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Old 28 October 2003, 04:10   #10
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Re: Searider propeller ventilation / cavitation - how to prevent?

Quote:
Originally posted by Searider
was charged over £200 to lift off, store and lift back on the engine when the transom was done.
You can buy an engine crane for less than that!

John
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Old 28 October 2003, 12:44   #11
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[QUOTE]Originally posted by Searider
[B]Rockey,

Is a lazer II better at running super cavitating / in aerated water or is it more suitable for giving me bow lift



yes to the above

try prop revs at poole cobbs quay. brian, and thomas,always
have lazers around,

what pitch prop are you running at the moment
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Old 28 October 2003, 13:05   #12
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Islander is selling an engine lifting bracket on this Forum for a 100 quid, seems a better option than carving your new transom.
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Old 28 October 2003, 13:16   #13
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You rite spellin cheker shot to bits...
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Old 29 October 2003, 02:59   #14
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more speed

Raise your motor 5 to 10cm.(watch your water pressure)
Buy a good stainless prop.(makes a big difference. try a mirage+
pitch 17-19)
Your problem has more to do with your prop than hight of your motor. Believe me I know.
A power lift is also a very good option.
Then you do at least 5-10 more knots.
greetings,
Niels

for more info niels@kroegie.nl
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Old 29 October 2003, 04:07   #15
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Quote:
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You rite spellin cheker shot to bits...
Yeah i am a bit of a chinscratcher when it comes to typing
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Old 29 October 2003, 04:09   #16
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quote:
--------------------------------------------------------------------------------
Originally posted by Searider
was charged over £200 to lift off, store and lift back on the engine when the transom was done.
--------------------------------------------------------------------------------

You can buy an engine crane for less than that!

or you could even hire an engine crane for a few days from a desent hire centre !!! after all what the hell do you want with an engine crane after your engine problems have been sorted out ?


Richard
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Old 16 February 2004, 16:56   #17
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Boat now sorted

Evening all,

Many thanks for the suggestions for how to sort my problem.

I have now had the transom top trimmed by 25mm and lowered the engine by one hole of adjustment - 20mm and the boat is back in speed demon hooligan mode.

Perhaps my new transom was just built higher than original.

My transom builder carried out this work at no charge and the charge for removing and refitting the engine was 1.5 hours on top of the cost of an engine service.

Now a happy bunny.

Duncan
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Old 16 February 2004, 17:53   #18
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Raising the engine too high can slow the boat down more because you can't raise the bow enough without the prop blowing out. A decent SS prop would probably allow you to run at the higher transom setting without losing bite. It also sounds like you might get a bit more top speed by shifting the CG back a bit so you don't have to run with so much engine trim to get the bow up for max speed. As ever, the downside of that would be that it might take slightly longer to get on the plane, however depending on your current setup this could be a good compromise, down to your personal requirements really.
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