Reply
 
Thread Tools Search this Thread
 
Old 01 July 2014, 16:42   #11
Member
 
Country: USA
Town: Portland, OR
Make: Zodiac F470
Length: 4m +
Engine: Suzuki DF25EL
Join Date: Jun 2013
Posts: 229
Quote:
Originally Posted by Projectile View Post
I'd have to say 'no' to that, but it all happens very quickly.

What were you getting at?
Sounds/behaves pretty much like mine when under-propped. You need either larger diameter prop (probably ideal solution but you won't be able to find one off the shelf) or higher pitch and/or 4 blade. Tach will tell story. As you speed up with WOT, prop has to slip more and more to provide increasing force needed to propel boat forward. As you reach critical region prop caviatates. Resistance on prop shaft drops, engine has access of power, races to highest RPM somewhere 6200+. Fortunately in EFI system it's all about fuel to RPM mapping. Regular Etec don't have mappings to create correct air-fuel mixture at higher RPMs. ECU trims fuel supply, power drops, prop grips water again. Cycle starts over.
__________________

__________________
SIBRider is offline   Reply With Quote
Old 01 July 2014, 18:44   #12
Member
 
Peter_C's Avatar
 
Country: USA
Boat name: Uh...a kayak?
Length: 4m +
Join Date: Dec 2009
Posts: 1,485
Quote:
Originally Posted by Projectile View Post
I'd have to say 'no' to that, but it all happens very quickly.

What were you getting at?
Think of it kinda like a caterpillar arching its back then continuing on. I have experienced that before on dead flat calm water, when loaded with three people and scuba gear. We tried moving things around and it never went away. It was quit evident though as you could feel the boat bow up and then release, at which point it would cavitate. I never have the problem when the water is even slightly bouncy as the boat releases the wave or whatever you want to call it.
__________________

__________________
Peter_C is offline   Reply With Quote
Old 01 July 2014, 18:55   #13
Member
 
Country: USA
Town: Oakland CA
Length: 3m +
Join Date: Feb 2005
Posts: 6,653
Quote:
Originally Posted by Projectile View Post
If my SIB hull is hooked, I'm screwed.
I think all SIB hulls as a whole have some degree of hook. You can't tauten up a flexible piece of fabric enough to keep it flat when you're throwing 20mph water at it.

jky
__________________
jyasaki is offline   Reply With Quote
Old 02 July 2014, 16:58   #14
Member
 
Locozodiac's Avatar
 
Country: Other
Town: Lima-Peru
Boat name: Nautile
Make: Sea Rider Boats
Length: 4m +
Engine: Tohatsu 18 /30 HP
Join Date: Sep 2007
Posts: 1,706
That's why you need to have tubes and keel top inflated to with gauge to achieve best water performance under flat and lightly choppy water conditions.

If your engine seats at 90 when sib is on plane, cavitates slightly when giving full throttle in straight line, much worst on tight coin turns, you neeed to lower that engine a bit untill cavitation disappears, only achievable under trial and error. If in need to chop transom down a bit as in Slate's example, it's bad musique and definitely you're not going for it.

Stay with that current delivered prop you need a engine/transom optimization not a prop maximization, you can do that afterwords if you like to, to take the max HP out of that engine.

Happy Boating
__________________
Locozodiac is offline   Reply With Quote
Old 02 July 2014, 18:16   #15
Member
 
Projectile's Avatar
 
Country: Canada
Boat name: WB465
Make: Zodiac
Length: 4m +
Engine: 2013 ETEC 30
Join Date: Dec 2012
Posts: 254
Quote:
Originally Posted by Locozodiac View Post
Stay with that current delivered prop you need a engine/transom optimization not a prop maximization, you can do that afterwords if you like to, to take the max HP out of that engine.
That may be, Loco, but numerous people here have had ventilation problems improve or disappear with different props. Why do you completely disregard these anecdotes? After all, a new prop is a simple fix, while chopping a transom is both complex and irreversible?
__________________
Projectile is offline   Reply With Quote
Old 02 July 2014, 19:27   #16
Member
 
Country: USA
Town: Seattle
Boat name: Water Dog
Make: Polaris
Length: 4m +
Engine: Yamaha 60hp
Join Date: Apr 2007
Posts: 1,152
I didn't realize your ventilation issue was on straight runs (unlike mine which was on turns).

I had a similar issue in my old SIB (12.5ft Bombard with Ali floor). But it only occurred when lightly loaded and I attributed it to cupping of the floor fabric. The hull would lift off the water momentarily then fall back down. Because I had a Yam with a 17" shaft on a "short" 15" transom the engine wouldn't ventilate or race at these times. The prop stayed firmly in the water despite the fact that the hull was rising up several inches.

Is there any way to lower your engine a little and see if it changes the behavior at all? If not I would wait for the Tiny Tach and see just how much racing is going on.
__________________
captnjack is offline   Reply With Quote
Old 05 July 2014, 23:23   #17
Member
 
Projectile's Avatar
 
Country: Canada
Boat name: WB465
Make: Zodiac
Length: 4m +
Engine: 2013 ETEC 30
Join Date: Dec 2012
Posts: 254
It seems that if I have an extra 250 lbs in my boat (in addition to me and a gas tank), she runs very well. No ventilation.

Does that indicate anything about the cause and solution?
__________________
Projectile is offline   Reply With Quote
Old 06 July 2014, 03:29   #18
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
What condition and age is the prop?

I suffered from ventilation/cavitation on my 5.4 Searider and cut down my (new) transom. Made no difference.

On closer inspection my ally prop had loads of fine cracks in it and must have been distorting under load. It was essentially worn out.

If there's ANY damage on your existing prop address that first.
Even if there's no damage apparent it might be worth having it inspected and balanced.
__________________
Searider is offline   Reply With Quote
Old 06 July 2014, 08:45   #19
Member
 
Country: Finland
Town: Helsinki
Boat name: SR 5.4
Make: Avon
Length: 4m +
Engine: Toh1 3,5 Yam 90/2S
Join Date: Oct 2009
Posts: 896
Quote:
Originally Posted by Projectile View Post
It seems that if I have an extra 250 lbs in my boat (in addition to me and a gas tank), she runs very well. No ventilation.

Does that indicate anything about the cause and solution?
Increased weight makes the boat sailing deeper and little slower, both things have an impact on prop ventilation. Think SIB boats are kind of challenging for the props, as the hull flex, the water stream coming towards the propeller(under the transom)is not as stable as on a hard boat. Also the shape of the area just before transom tend to be different(not optimal) than on hard boats.

I would borrow and test a loads of different props, different brands. Can be huge different on two equal pitched props by different brands. On my SR5.4 a K series 19 pitch works fine but a black max 19 is no good, as the actual pitch is much higher than on the K series so it wont rev enough.
__________________
fun on a boat is inversely proportional to size...sort of anyway
C-NUMB is offline   Reply With Quote
Old 07 July 2014, 00:08   #20
Member
 
Projectile's Avatar
 
Country: Canada
Boat name: WB465
Make: Zodiac
Length: 4m +
Engine: 2013 ETEC 30
Join Date: Dec 2012
Posts: 254
Quote:
Originally Posted by Searider View Post
What condition and age is the prop?
Good point, but my prop is mint, hardly used, OEM aluminum. I bought the motor brand new, out of the crate, and these particular small engines come with a prop from the factory. She's probably got < 30 hours on her.
__________________

__________________
Projectile 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 15:59.


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