Reply
 
Thread Tools Search this Thread
 
Old 27 October 2003, 11:58   #1
Member
 
Country: UK - England
Town: Southampton
Boat name: SMH Rib / War Shot
Make: Ribtec / Scorpion
Length: 4m +
Engine: 10hp Yam / 150hp opt
Join Date: Sep 2003
Posts: 1,894
RIBase
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
__________________

__________________
Searider is offline   Reply With Quote
Old 27 October 2003, 13:26   #2
Member
 
Country: UK - Wales
Town: Milford Haven
Boat name: Various
Make: Commercial
Length: 10m +
Engine: Screw / Voith / Jets
Join Date: Sep 2002
Posts: 791
Send a message via MSN to Alex Brown
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
__________________

__________________
Flickr Photos
Youtube Videos
Alex Brown is offline   Reply With Quote
Old 27 October 2003, 13:39   #3
Member
 
Country: UK
Join Date: Oct 2003
Posts: 27
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
__________________
rockey is offline   Reply With Quote
Old 27 October 2003, 14:46   #4
Member
 
Richard B's Avatar
 
Country: UK - England
Town: Devon
Boat name: White Ice
Make: Ranieri
Length: 5m +
Engine: Suzuki 115hp
Join Date: Jul 2002
Posts: 5,015
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?
__________________
Richard B is offline   Reply With Quote
Old 27 October 2003, 14:58   #5
Member
 
Country: UK - England
Town: SOUTHAMPTON
Boat name: Won't get Fooled Again
Make: Ribtec
Length: 6.5
Engine: Honda 130
Join Date: Mar 2002
Posts: 888
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
__________________
I need a little bit of Rhythym and a little bit of Blues
thewavehumper is offline   Reply With Quote
Old 28 October 2003, 04:04   #6
Member
 
Country: UK - England
Town: Southampton
Boat name: SMH Rib / War Shot
Make: Ribtec / Scorpion
Length: 4m +
Engine: 10hp Yam / 150hp opt
Join Date: Sep 2003
Posts: 1,894
RIBase
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
__________________
Searider is offline   Reply With Quote
Old 28 October 2003, 04:09   #7
Member
 
Country: UK - England
Town: Southampton
Boat name: SMH Rib / War Shot
Make: Ribtec / Scorpion
Length: 4m +
Engine: 10hp Yam / 150hp opt
Join Date: Sep 2003
Posts: 1,894
RIBase
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
__________________
Searider is offline   Reply With Quote
Old 28 October 2003, 04:12   #8
Member
 
Country: UK - England
Town: Southampton
Boat name: SMH Rib / War Shot
Make: Ribtec / Scorpion
Length: 4m +
Engine: 10hp Yam / 150hp opt
Join Date: Sep 2003
Posts: 1,894
RIBase
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
__________________
Searider is offline   Reply With Quote
Old 28 October 2003, 04:41   #9
Trade member
 
Andre's Avatar
 
Country: UK - England
Town: West Sussex
Boat name: Grey Mist
Make: Parker 800 Baltic
Length: 8m +
Engine: 2xMercury Diesel 170
Join Date: Oct 2002
Posts: 3,451
RIBase
Send a message via Skype™ to Andre
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
Andre is offline   Reply With Quote
Old 28 October 2003, 05:10   #10
Administrator
 
John Kennett's Avatar
 
Country: UK - England
Town: Brighton
Length: 3m +
Join Date: May 2000
Posts: 6,829
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
__________________

__________________
John Kennett is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 06:51.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.