Motor for raft - Page 4 - Mountain Buzz
 



Reply
 
Thread Tools Display Modes
 
Old 05-17-2019   #31
 
Join Date: Feb 2005
Posts: 995
Hi Cgilbane,

Maybe my use is not precise, but it's close, I think.

Cavitation -- "The formation of an empty space within a solid object or body. The formation of bubbles in a liquid, typically by the movement of a propeller through it."

Under some circumstances, cavitating fluid can cause damage to the surfaces it interacts with -- the damage to the concrete tubes at Glen Canyon Dam in the 80s flood is a macro example. When the bubbles collapse, there is a shock wave that can damage the solid surface it is in contact with.

While a fast prop can cause cavitation on it's own, I'm referring to running a fast prop in an inadequate or unstable flow -- like at or near the disturbed surface behind the moving raft hull or in the aerated water of a rapid. The prop spins faster and with less propulsive effect when it is spinning against the aerated flow, as opposed to solid water.

If that doesn't meet the classic definition of cavitation, hopefully this is a better explanation of the effect I'm talking about.

Have a good one.

Rich

richp is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 05-17-2019   #32
 
Black Hawk, Colorado
Join Date: Mar 2018
Posts: 53
Quote:
Originally Posted by richp View Post
Hi Cgilbane,

Maybe my use is not precise, but it's close, I think.

Cavitation -- "The formation of an empty space within a solid object or body. The formation of bubbles in a liquid, typically by the movement of a propeller through it."

Under some circumstances, cavitating fluid can cause damage to the surfaces it interacts with -- the damage to the concrete tubes at Glen Canyon Dam in the 80s flood is a macro example. When the bubbles collapse, there is a shock wave that can damage the solid surface it is in contact with.

While a fast prop can cause cavitation on it's own, I'm referring to running a fast prop in an inadequate or unstable flow -- like at or near the disturbed surface behind the moving raft hull or in the aerated water of a rapid. The prop spins faster and with less propulsive effect when it is spinning against the aerated flow, as opposed to solid water.

If that doesn't meet the classic definition of cavitation, hopefully this is a better explanation of the effect I'm talking about.

Have a good one.

Rich
Thanks for helping me understand, Rich. My knowledge of this stuff comes from basic definitions I was googling. This is helpful.
Cgilbane is offline   Reply With Quote
Old 05-17-2019   #33
 
Black Hawk, Colorado
Join Date: Mar 2018
Posts: 53
Quote:
Originally Posted by richp View Post
Hi,

I used a 20 inch shaft Honda on my NRS E-140, with no problem keeping the prop in the water.

A 25 incher might cavitate a bit less in fast water, but for purely flat water use with normal diameter tubes, I would judge 20" would be fine.

FWIW.

Rich Phillips
What do you use for an extender to reach your tiller from your chair?
Cgilbane is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 05-17-2019   #34
 
Join Date: Feb 2005
Posts: 995
Hi,

Should have added that the 20" lower unit was plenty long for me on the 14' NRS in the picture. Although you can't see it well in the photo, the transom allowed some vertical adjustment, but really was not needed.

I also used that motor on a 16' NRS cat, and you can see it on the El Tigre in the picture, which had 30" tubes. Never felt a need for a longer lower unit, even with tubes that tall.

(I always thought the 25" lower units were primarily a sailboat product. That may or may not impact resale some day.)

I might worry a bit about a 25" lower unit tapping bottom/rocks under some circumstances. But you likely could raise the transom if that became an issue.

FWIW.

Rich
Attached Thumbnails
Click image for larger version

Name:	P1010127_1558135218299.JPG
Views:	31
Size:	629.6 KB
ID:	35037  
richp is offline   Reply With Quote
Old 05-17-2019   #35
 
Join Date: Feb 2005
Posts: 995
Hi,

I chopped the end off an old bent Carlisle oar, cut four slices 3"-4" longitudinally up from the cur end, spread them a little, and used a hose clamp to tighten it up on the tiller handle. The right sized PVC pipe probably would work too, but the old oar had a conveniently sized padded grip.

Extensions work fine in flat water. But if you are in moving water and may need to pull the motor quickly, you are that much farther forward from the power head, which sort of slows you down.

Hope this helps.

Rich
richp is offline   Reply With Quote
Old 05-17-2019   #36
 
Salt Lake City, Utah
Paddling Since: 1987
Join Date: Sep 2012
Posts: 129
When pushing multiple rafts a high thrust propeller should be used. They are used to reduce the load on the engine = your engine will last longer. Sure they run fine on the standard prop but you should use a high thrust prop.

In my experience a 4 hp does just fine pushing 4-5 rafts and 6hp is more than enough for a large barge of rafts. 8hp+ have 2 cylinders and weigh a bunch more. Like others have said there are diminishing returns on power/speed

Motors come in 3 shaft lengths. Short (15), Long (20") and extra long (25") (sometimes called sail). A short length will work if you use a lifting plate and a tiller extension, otherwise a 20" would be the preferred shaft length for most rafts.

Some small outboards come equipped with charging. This can be used to charge 12V batteries. They are typically around 5A and it is a common feature on the "sail" models.

If you are running rapids a 4 stroke outboard can be tipped up using the "tilt" position, but if it is removed and laid down you must follow the directions for the motor to store in the proper position. 2 stroke motors store any way but prop over power head

I use a 2-stroke 15"shaft with a lifting plate and just lift the plate up to the highest position for the rapids. This keeps the motor completely out of the water. When I use the tilt position it sometimes falls down.

We use a piece of PVC pipe with a couple slits in it to go over the handle. I also hooked a switch into the end of the handle and attached it to the kill switch in the motor to make it easier to kill the motor in case we hit the bottom or have some type of emergency.

If propane was a must I'd go with a Tohatsu motor. Tohatsu has a good reputation and makes most small gasoline outboards. I believe only the carburetor needs to be different on a 4 stroke motor
unlucky is offline   Reply With Quote
Old 05-18-2019   #37
 
Salida, Colorado
Paddling Since: 1983
Join Date: Nov 2015
Posts: 188
Quote:
Originally Posted by Cgilbane View Post
What kind of boat do you have? What length motor did you get, the 20 or the 25

I bought the 25 inch, I used it once so far, on a 16 foot cat with a cabella's adjustable transom, but mounted it on my 18 foot Briggs Grand Canyon dory to see if the shaft was long enough, and it fit perfectly (the dory has a removable transom piece to allow the motor to mount a foot lower)


I hung it on the back of my Avon Pro with the standard height "Beckett" motor mount that was designed to hang off the rubber motor becketts that early Avon's came with from the factory. It looked a tad long, but you have to use the trim adjustment anyway as the motor hanging off the back of rubber tends to push the motor under the boat to some degree anyway.



The motor was purchased for the dory, and that will be it's primary use. While a 20 inch would likely work, due to the pitching and rolling that's prevalent with a dory, I was worried about the cavitation Rich P speaks about. I disable the hold down mechanism on outboards in river so as to minimize the chance of the lower end hitting a rock HARD, have motored a lot of miles over 30++ years of boating and only lost 1 prop using this method. I used to use a circular motor guard made out of cast aluminum that encircles the prop, but I sheared that off multiple times, and finally gave up. It hung another 3 inches below the prop's lowest point and there's no really good way to attach it to the motor.



For a tiller extension I use a busted cataract shaft about 36 inches long, with a plastic sleeve with slits cut longitudinally in it, and thumbscrew hose clamps over the sleeve to secure to the handle of the motor.
MNichols is offline   Reply With Quote
Old 05-18-2019   #38
 
Black Hawk, Colorado
Join Date: Mar 2018
Posts: 53
Wow, thank all of you so much. You have beyond answered all my questions. Ordering the motor today.
Cgilbane is offline   Reply With Quote
Reply


Thread Tools
Display Modes

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


Similar Threads
Topic Thread Starter Forum Replies Last Post
Raft Motor...What's the best? beanack Whitewater Rafting 37 08-25-2014 03:36 PM
Motor on 18 foot raft - self bailer Yukon77 Rafting | Gear Talk 13 05-03-2013 03:40 PM
Raft motor sward Rafting | Gear Talk 15 08-03-2011 06:59 PM
Raft Motor Mount Methods nicho Whitewater Rafting 10 09-27-2009 02:26 PM
what size/style/brand motor for a 14' raft? JennMiko Whitewater Rafting 4 08-23-2008 09:28 AM

» Classified Ads
Powered by vBadvanced CMPS v3.2.3

All times are GMT -6. The time now is 03:33 AM.


Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.