Which Elephant Ear prop for a Tohatsu nsfA3 9.8HP Ultra long shaft on 22' Walkaround?

dazk14

Ensign
Joined
Jul 22, 2008
Messages
966
I'm rigging a "Get a Customer Home Motor" for a Seaswirl 2150 O/B Walkaround (4k loaded)and want to get him setup well, should the need arise for its use (ie: O/B croaks)

It is a four stroke 2007 9.8HP. It will NOT be used for trolling.

I believe the choices are a 5" and a 7" and am I correct that the Tohatsu and Solas are identical, although the iBoats picture appears otherwise.

It'll be used in open water in the Boston area ocean, so there could be winds and waves.

I know a few of you guys have been around the block with this, so if you have a moment, please weigh in. Thanks!
 

pvanv

Admiral
Joined
Apr 20, 2008
Messages
6,500
Re: Which Elephant Ear prop for a Tohatsu nsfA3 9.8HP Ultra long shaft on 22' Walkaro

Yes, they are the same props.

You will likely want the 3B2B645120 (7-pitch). But it's difficult to say with 100% certainty, because the Seaswirl has a completely different underwater design from a heavy sailboat, which is where I have all my EE prop experience. Remember that you will always be in displacement mode, and severely underpowered (just like a sailboat), so minor pitch differences will make huge RPM changes. Still, the 5 pitch might over rev. on that boat. I would start with the 7 pitch, and do on-water testing with a tach to confirm. Unless the WOT RPM is below 4,800, the 7 pitch will probably be the best choice.

For reference: On my personal 28-foot 7,000+ pound sailboat, the 7 pitch never comes up to proper WOT RPM (maybe 4900), so I'm running the 3B2W645110 (5 pitch), and get just slightly above 6,000 RPM on the factory tach (not quite engaging the ESG of the NSF9.8A3) under ideal sea conditions. I probably could have the local prop shop add a touch of cup, or try to increase pitch a half inch. Interestingly, the boat's knot meter shows nearly the same speed, regardless of the prop I run (maybe 1/2 knot faster with the 7 pitch), so I am probably hitting the wall re: HP limit of the motor vs drag of the boat. Likewise, on lighter sailboats, say a Catalina 25 (4,000+ pounds or so), the 7 pitch works best.

Now of course when you add in wind and waves in an "I need to limp home" situation, you are no longer under ideal conditions, and boat speed will be lower -- and so will the NFS9.8A3's WOT RPM. So... as long as the 7 pitch gives a WOT RPM above 5,000 under ideal sea state, that would be the prop I would select; If it's below 4800, switch down to the 5 pitch, even though in completely flat water it may be possible to slightly exceed proper WOT RPM if an idiot operator decides to "floor it" on a calm day.
 
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