Reply
 
Thread Tools Search this Thread
 
Old 02 April 2004, 16:06   #1
Member
 
Hightower's Avatar
 
Country: UK - England
Town: Fareham
Length: 6m +
Join Date: Sep 2003
Posts: 7,855
Prop. height

Put quite simply. How do you know what position your prop should be in relation to the hull? Trial or error!

Everytime I walk pass my Directors, Honda 4stroke race boat (about 20 times a day), I can't help notice that the prop and cavitation plate look way too high. The prop is above the bottom of the transom, this can't be right! Can it?

Andy
__________________

__________________
Andy

Looks Slow but is Fast
Member of the ebay Blue RIB cover club.
Hightower is offline   Reply With Quote
Old 02 April 2004, 16:14   #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
Sounds like a surface piercing prop setup.

I know very little, but I'm sure there are others on this forum who can explain, either that, or look on google or something:-

http://www.google.com/search?q=surfa...UTF-8&oe=UTF-8

-Alex
__________________

__________________
Flickr Photos
Youtube Videos
Alex Brown is offline   Reply With Quote
Old 02 April 2004, 16:26   #3
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: Prop. height

Quote:
Originally posted by Hightower
Honda 4stroke race boat (about 20 times a day), I can't help notice that the prop and cavitation plate look way too high. The prop is above the bottom of the transom, this can't be right! Can it?

Andy
Is it attached to a jacking plate? If so, it can raise/lower the engine. I don't know if these are used on Honda 4st or not.
Richard B is offline   Reply With Quote
Old 02 April 2004, 17:34   #4
Member
 
Pete7's Avatar
 
Country: UK - England
Town: Gosport
Boat name: April Lass
Make: Moody 31
Length: 9m +
Join Date: Aug 2001
Posts: 4,850
Quote:
Trial or error!
Yup. Start with the anti cavitation plate about 1 inch above a line that would extend out from the base of the rib hull. If this works great, and if you want to try and go faster then raise it a hole.
If the prop spins and looses grip then its to high. S/S props allow you to run a higher prop and in theory faster but the prop will ventilate more as you go higher particuarly when making sharp turns or zooming across the waves. If youwant to go really fast buy one of those speed boat thingys.

Long shaft engines are built for a 20" transom. Opsrey knowing you would start at 1" up decided to build 21" transoms on there ribs to make life easy for owners.

Pete
__________________
Pete7 is offline   Reply With Quote
Old 02 April 2004, 17:39   #5
Member
 
Hightower's Avatar
 
Country: UK - England
Town: Fareham
Length: 6m +
Join Date: Sep 2003
Posts: 7,855
No, it's not on a jacking plate.

The race series this year are running 150hp units that seem to be higher than the 130hp ones from last year, but that said the 130hp outboard sat quite high as well.

I guess what I really wanted to know was what height is the correct one?

Surface piercing props.......are they used when you can't get the Engine any lower, or do they add any special significance to the performance of a set up?
__________________
Andy

Looks Slow but is Fast
Member of the ebay Blue RIB cover club.
Hightower is offline   Reply With Quote
Old 02 April 2004, 18:05   #6
Member
 
Pete7's Avatar
 
Country: UK - England
Town: Gosport
Boat name: April Lass
Make: Moody 31
Length: 9m +
Join Date: Aug 2001
Posts: 4,850
Think you need someone like dirk to answer that one, but they are available for your Yamaha.

Pete
__________________
Pete7 is offline   Reply With Quote
Old 02 April 2004, 18:22   #7
Member
 
Hightower's Avatar
 
Country: UK - England
Town: Fareham
Length: 6m +
Join Date: Sep 2003
Posts: 7,855
Pete,

you know that we talked about the fact my prop seems to be oversize (pending mechanical/electrical issues) the other week. Someone I was speaking to the other day said that if the engine was raised, that would increase the revs. Do you think that's right? Perhaps he was thinking of surface props!

Need to concentrate on getting Kohaku into the water first though.

Andy
__________________
Andy

Looks Slow but is Fast
Member of the ebay Blue RIB cover club.
Hightower is offline   Reply With Quote
Old 02 April 2004, 18:30   #8
Member
 
Pete7's Avatar
 
Country: UK - England
Town: Gosport
Boat name: April Lass
Make: Moody 31
Length: 9m +
Join Date: Aug 2001
Posts: 4,850
I think you would be better off with a 2" less pitch prop to get the revs up. this will increase the acceleration stop overloading the engine. If you raise the engine to far the its a pain to drive as it will keep loosing grip and you have to stop and accelerate again. Been there done that then put the engine back to were it should have been in the first place.

Pete
__________________
Pete7 is offline   Reply With Quote
Old 02 April 2004, 18:39   #9
Member
 
Hightower's Avatar
 
Country: UK - England
Town: Fareham
Length: 6m +
Join Date: Sep 2003
Posts: 7,855
Really just a case of getting my ass in gear, having some nice weather and get all that work done. My Easter deadline is not looking too good at the mo. Will keep you informed.

Andy
__________________
Andy

Looks Slow but is Fast
Member of the ebay Blue RIB cover club.
Hightower is offline   Reply With Quote
Old 03 April 2004, 02:47   #10
Member
 
Cookee's Avatar
 
Country: UK - England
Town: Salcombe, Devon, UK
Boat name: BananaShark
Make: BananaShark
Length: 10m +
Engine: 2xYanmar 260 diesels
Join Date: Mar 2003
Posts: 4,225
Quote:
Originally posted by Hightower
Pete,

you know that we talked about the fact my prop seems to be oversize (pending mechanical/electrical issues) the other week. Someone I was speaking to the other day said that if the engine was raised, that would increase the revs. Do you think that's right? Perhaps he was thinking of surface props!

Need to concentrate on getting Kohaku into the water first though.

Andy
The Honda props are definitely not surface piercing props although most props can work quite high on the transom.

Be very very carefull when raising engines high - you must have sufficient water pressure for the engine to cool - Race engines have the water pickup mounted on the nose of the gearbox - leisure engines get thier water from the side of the gearbox several inches higher!

Raising the engine will give you more revs if it is under revving before, and will probably make you go faster as well. It will usually give more slip on acceleration so won't be as good for skiing for instance!

We run lifters on the raceboat because of the benefits, but we will only fit them on the leisure boat by request as they are very expensive!
__________________

__________________
Cookee
Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
Cookee 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 01:43.


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