pithy proposals on Prop pitch please

I have a 30 foot Pearson Wanderer with fresh bottom paint ( Warp Drive, Photon Torpedoes and retractable landing gear too) and I am constructing an outboard well just forward of the transom, into which a 9.9 horsepower Johnson sailmaster will go. The scent of 2-stroke fuel and exhaust is 5,098,362,000 times more appealing than that of diesel, to me. I am fully aware of the increase in the probability of going Supernova by the way.. I may throw on a 15 horsepower carburetor just for a little bit of extra reserve power for going into Heavy Seas or alien force beams. Solace has two 10 inch diameter props, one being 7 inch pitch and the other being 5-inch pitch. Both have four blades,... though they do try to upsell you on the upgrade which is 4.1 blades.. I am not certain, but I do not believe the sailmaster used a different gear ratio than a regular 9.9. I was just about to flip a coin, heads being for the 7-inch pitch and Tails being for the 5-inch pitch, of course, and then thought "why not seek out wisdom from this fine Consortium of the continually calculating collection of the engineering minded, vodka Collective? " I apologize in advance, as I ran out of my anti alliteration medicine. Thank you.

$500,000 and a free trip to Hawaii to the person who has actual experience with a setup very similar to this.

 

how many blades?

 

4 blades

 

I seem to recall using a 9" pitch on a 9.9 Johnrude as a kicker for a 21' powerboat, 5" sound more like a fan than a propellor. And looking to change the carb makes little sense.

 

Given that Miss Margaret is a displacement hole with a water line length of just over 23 feet, the boat will never go more than 6.4 knots in normal usage. Five inches of pitch is pretty close to a fan blade though. I really would prefer to keep the motor 500 RPM shy of its maximum just for longevity sake. The 9.9 is rated at 5500 RPM and the 15th is rated at 6500 RPM. I would not want the motor to run over 6,000 RPM. If I knew, with Precision, the efficiency of the prop the calculation is actually rather simple.
Down to the last part number the 9.9 and 15 are identical Motors except for the carburetor and in some years the addition of a tiny shim between the Reed plates and the stops for the read, and an exhaust two for a couple years that was only on the 15th before it was deployed on the 9.9. There are several years where the two Motors are identical accepting for a carburetor with a larger Venturi. Amongst people familiar with the motor, it is a common and easy swap and what increase my horsepower 40% at a minimum.

 

Starting at the beginning, the most important issue by far is getting the prop deep enough. You can set the OB very deep in the well, plus you can buy extension shafts and spacers to convert a 25"er into a 30"er - and I recommend both of these.

With a slow boat, you want a zingy prop, not a big efficient one. The reason is that the boat motions represent massive perturbations to the inflow of the prop. Using a high slip setup vastly reduces the effect of these perturbations and gains you more than you lose in all but mill-pond conditions. A stock three-bladed prop has plenty of blade area for what you are trying to do. Dropping down to a two blader is an option, but will play havoc with reverse. I'd go with the stock prop.

I ran a Cal 28 with a Merc 15 in a well down in the Florida Keys. It had a 30" shaft and sat with the cowl at the waterline when doing 5 knots. The stock prop (possibly 9.25 x 9) worked fine. Basically, you want to be able to develop full bollard pull, so you want to be able to hit the power band rpm while tied to the dock. This usually works out to around 25 fps or more slip for pusher outboards with stock props. For a 4 blader, you might need a 5 pitch prop to get into the power band, but I think this would be a really annoying prop to live with on a recreational boat. The engine would hunt around with every little wave and boat movement. Power output would vary all over the map from second to second. And the hull wouldn't be able to find it's sweet spot the way it can with a slippier prop. Grinding along like a cog railroad just isn't a good thing for small displacement cruiser.

So I would do the following -

1. Mount a 30" shaft extension kit.

2. Test stock prop to see what kind of rpm you can hit tied to the dock.

3. Do a couple crash stops from 4 knots fwd to 4 knots reverse and see if you can keep the boat in a straight line. This will require huge rudder inputs when still going forward, but the idea is you should be able to back down the same path you came in on. A flatter prop can sometimes help with this, so if things get too crazy, drop down to a 7" pitch prop. I never was quite satisfied with reverse in my Cal. Putting her in the slip in a 20 knot tailwind was quite an adventure.

 

Yes I do believe it would take a bit of Rudder in put! Haha. I am reconfiguring the rudder from The Barn Door hung on the Keel factory-style to Ana CA 00124 file with the Pivot Point 20% behind the Leading Edge. I have gotten along so far with a 7 & a half horsepower 4 stroke Honda on one of those hideous little Contraptions on the back articulating parallelogram I think is the fancy name for it. The motor got nowhere near its RPM band that I knew it was a temporary deal so I did not care that much. I am still not 100% sure that I am not going to use a Yanmar 3gm30 I happen to have. It puts out 24 horsepower but the propeller is severely surface pitted and I am certain I would lose a good bit of the efficiency just from that. I do not care for motor sailing that much and so the fuel economy difference is not such an issue to me. I would be able to sell the NR and get my boat out of the marina sooner rather than later. Plus I would have all that space where the stinky diesel once resided. The first thing I thought about when contemplating not putting the diesel and was a well to solve a lot of the mischief caused by that it is Contraption on the back. Thank you very much for the input I will definitely keep the factory prop on and do some experimenting. One of the nice things about the outboard is that it is something I am very familiar with having owned several at the same lineage and changing a prop is easy as is rebuilding the whole thing in the cockpit with a couple hundred dollars worth of bits. I have a 15 shirts at which is electric start and so I will put the 15 card on the sailmaster and relish its 25 inch shaft stuck deep into the water. Having the dinghy and Main Mothership motor being the same makes having stairs a lot easier. Hack I probably have enough bits and pieces to have a good powerhead wrapped up in heavy plastic bags sitting and ready to install. The one slight disadvantage is alternator output. Fortunately I have two 300 watt solar panels. Thanks again for the input

 

One thing which does occur to me is that the slippery system would reduce fuel efficiency. Do you care to gas on that difference? At some point fuel efficiency does matter though.

 

Not at this point it doesn't. You'd have to really plan ahead if you wanted to burn more than $10 of gas pushing that boat in one day. And I'm sorry to hear about the ob bracket ordeal. Been there - done that. Helped a friend shift a Newport 30 which he was given for free on condition he could get it out of the marina by dark. We stripped my motor out of the Cal and used it to motor all night. It was blowing hard and we were going backwards at times, and I ended up towing and yawling with my dink and it's 3 hp for 6 hours. Even with a 30" shaft, the ob on the bracket was out of the water more than it was in the water, and we about wrung the bracket off the boat.

Realistically, I sailed the Cal about twice a week for two years, I had to navigate a marina and a couple of creeks under power. I bought about a gallon of gas a month.

 

You are kidding yourself trying to turn it in to a 15, even assuming the difference is only in the carb ,might be differences elsewhere such as porting and timing etc, but regardless that basically allows the engine to rev out further, you don't run the motor at peak revs anyway, so not worth worrying about.

 

You are incorrect in a couple of ways. In the first place I have years and years of experience with these motors and I have checked part numbers as have many other people who play with these specific Motors. The Block & head and pistons and every single thing else has the same part number except for the aforementioned components. Secondly, as someone who used to drag race and who has seen quite a few dynamometer runs of engines with various things changed I can tell you that a larger carburetor does not only Supply more horsepower by allowing an engine to rev higher but also supplies better cylinder filling at lower RPMs which means more horsepower. It is really simple physics if you have less restriction at a given RPM 4 Fuel and air to flow into the engine you will get a higher degree of cylinder filling. Obviously at 1000 RPM when the throttle is wide open one is not going to see a difference between the small and large carburetors. But by 3000 RPM or so there is a marked difference. There would certainly be a difference in the 4,000 to 5500 range which is what I am concerned with. It is actually a pretty common modification in areas such as Ontario or Minnesota which has many lakes which are horsepower restricted. People bought the 9.9 and put on the bigger carb. At some point of course there is diminishing returns. Putting a thousand CFM 4-barrel carburetor on a 4-cylinder engine isn't going to get you any more power then 500 CFM or so depending on how much camshaft head porting and other things have been done to the motor..

 

Forget all that, worry about the prop, anything less than 7" sounds crazy to me, if it is the 2.66 to 1 box.