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-   -   Outboard propeller guard- Ideas are welcome (http://www.sailnet.com/forums/gear-maintenance/59255-outboard-propeller-guard-ideas-welcome.html)

pedcab 10-26-2009 07:19 AM

Outboard propeller guard- Ideas are welcome
 
Hi everyone

Since Fulô is ashore for the mid season maintenance I think it is time to once and for all solve an issue that has been scaring me.

As on many trimarans, Fulô's transom is just to small to fit an outboard on the traditional bracket mount and as such my engine rides sideways, attached to the port side of the main hull.

This configuration, as you can imagine, represents a very real propeller injury threat to anyone who falls overboard in the space between the main hull and the portside float while the boat's moving ahead.

To reduce the danger of serious injury to any crew that sails with me or to my mate Neil (the dog) I thought of fitting a propeller guard.

Following this I've been searching for ideias on the net and I'm inclined to fit a ducted propeller guard instead of the most commonly found cage like protection, thus certainly improving in safety and also expecting some added performance from the engine due to the ducted fan principle.

http://img193.imageshack.us/img193/6...ellerguard.jpg

Does anyone have any thoughts or experience arround this subject?

Many thanks in advance

Pedro

xsboats 10-26-2009 08:38 AM

Pedro, there used to be a prop guard called the Handler that was manufactured for use on OMC outboards. While I never bought one, I was intrigued by the principle, and molded one in fiberglass. I mounted it on an old Seagull that I had kicking around the shop.I chose the Seagull for it's squared off tips on the prop. The handler came with a 4 blade square tipped prop to replace the buyers existing prop. The primary function of the handler and my copy was to eliminate the vortex formed at the end of each prop tip. The result was a significant increase in thrust and a slight decrease in topend speed. The loss of speed was only noticed on my dinghy and not my boat.The sides of the guards were not vented as is yous illustration. I have also bought and used similar units from military surplus auctions that were mounted on 25 and 135 hp Johnson outboards. I think this bladeguard would be a great addition to your boat,but I would research as to whether the slots are really beneficial or not.

pedcab 10-26-2009 09:25 AM

From what I've searched about ducted fans the slots are not beneficial at all and I'm not planning to include them on the duct I'm planning to mould for my boat.

Many thanks for your thoughts! It's just how I imagined it would be.

As far as I'm concearned your experience had very logic results when you talk about increase in thrust and slightly reduced top speed in your dinghy.

More thrust is obviously an advantage on an auxiliary outboard while a bit top end speed loss (which I suspect might have been caused by the extra drag) is something I'm willing to accept, although it may not even be noticeable on a boat travelling at 7,5 knots (Fulô's speed with its 5hp tohatsu at 3/4) because, as you surely know, the dinamic drag coefficient increases with speed and it is my conviction the the drag induced by the duct will only have sensible negative results at planning speeds...

Besides all this I also expect some reduction in the propeller aeration issues I experience very oftenly because of my offset engine setup being extra sensitive to boat heel (even on a trimaran)

Once again, many many thanks for sharing your precious experience!!

PS- I intend to build my duct out of wood/epoxy/fiberglass laminate to avoid bimetal electrolyses to arise from the assembly... Any alternatives?

tommays 10-26-2009 09:33 AM

There and off the shelf item at places like Internet outboard But there safety record is dubious as it is still easy to stick a dangling had of foot into them if you were struck

pedcab 10-26-2009 10:02 AM

Tommays:

I understand that, thank you for your opinion, and I've also read about it, but I'm sure that to use a prop guard of any kind is always better than to keep on going without one on such an accident prone configuration as mine is...

I've also considered the cage like prop guard, but those have induce awfull cavittion and drag...

Here's a picture for you to better understand Fulô's engine setup

http://img203.imageshack.us/img203/8952/motorfulo.jpg

PS- Unfortunately "off the shelf" also means "off the wallet" and I'm afraid that I'm really going to have to resume to DIY methods :)

sailingdog 10-26-2009 10:06 AM

You might want to see if a RINGPROP is available for your outboard. The RingProp was specifically designed to reduce the risk of injuries by propellors.

pedcab 10-26-2009 10:51 AM

Thanks Sailing Dog. All ideas are welcome to the undergoing R&D process :)

Returning to bimetal corrosion, take a look at what I´m afraid of (I found this pic after some deeper research:

http://i679.photobucket.com/albums/v...edcab/prop.jpg

Note the corrosion next to the mounting bolts of the stainless steel (or so it seems) guard moundted on the aluminium alloy outboard... At least this guy could have had its electrolyses protection reinforced......

To avoid this, mine is going to be made out of some non metalic home made compound...


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