TOPIC: Extension for the lower

The prop on my '63 100 hp cut loose much to easy. I'm running a three blade 21 pitch on a '62 18 foot Thompson that with 2 people people and gear weighs no more than 1500 lbs (excluding the motor).

I frankenmerc'd an eighties PTT onto the Merc so that, in addition to the huge improvement in drivability all around, would allow me to bring the bow up under speed for a better plane. But that makes it worse with anything other an a mild trim up.

I've always wanted to get the prop deeper, and I think putting on the PTT actually raised it up slightly (1 inch or so?)

Long way to go to get to the question. I know Merc made extensions to convert a short shaft to a long, but did they make any that were just one or two inches?

Here is a frame from a video - shot poorly. This was at probably 3000 RPM with a milt trim up

Only ones I've ever seen are 5".

Another option would be a manual jackplate mounted lower than normal to bring the cavitation plate even with the keel. www.bobsmachine.com/Products/jackplate_convertible.cfm

Hi Peter,

Do you have any cup on that prop? I did a lot of prop testing on my G3 with its '63 850. The cav plate at that time was 2.5" above the bottom of the boat. Non-cupped Michigan and old Merc 2 blade props always cut loose on corners and in a straight line with just a moderate amount of trim. I switched to cupped aluminum quicksilver props and they held the bite really well... could turn them hard and over trim and were still good.

I was also wondering about your pitch? 21P seems pretty tall. Even with the 1000 mods I've done to the 850, that's too much prop for my 14' G3. I'm running a 19P cupped stainless prop these days. The boat is 400 lbs, with me and gear I'm probably about 750 lbs, motor adds about 240 more... so I'm at about 1000 lbs gross weight.

Dean

Thanks all!

Tim, I thought I had seen a short extension in my travels but alas you would know better than I - I'd rather find a different solution.

Kerry thanks for the link! I've been looking for a jack plate such as that. I'll keep it on mind.

And Dean,
You describe just the problem I have. Prop cuts loose in turns, with even mild trim, and if I gas it. Will even cut loose if I am plowing straight into a series of steep wake.

I run a 21P to get some bite. The motor seems to really like that pitch even with a full load of peeps. The two blade brass that came with the motor is a 21 pitch, and it cuts loose easier than the 3-blade. I've run a 17 and a 19 (3 blades) with the same results.

I was actually thinking along the same lines as you - a cupped prop or even a 23P. Starting the search now to see if I can something affordable in a 21P cupped 3 blade.

Peter, what's your full throttle RPM? That should be the determiner for pitch choice. As Dean indicated, that's an awful lot of prop for a 100hp to pull, unless you've installed a later-model lower unit with 2.3:1 gears. Doubtful as you'd probably know that, it requires quite a few mods to fit a newer L/U on the old 100hp!

Dean's spot-on as far as prop-cupping is concerned, it'll definitely hold better. You'll be able to pick a lower-pitched prop for better hole shot, while still retaining most if not all of your top speed.

On my old 1350 I-6 for a while I ran a 4-bladed plastic Comprop which was extremely smooth, and had fantastic holding power in turns. It was almost impossible to get it to ventilate.

With 4 blades, there are more blades actually biting water at any given time, compared to a 2 or 3-bladed prop. The old 2-bladed brass props are definitely not "hi-performance" in that regard!

There are a lot of 4-blade prop choices besides the plastic Comprop; Michigan Wheel (for one) has 'em in aluminum and stainless.

I bet on a relatively low-performance hull such as yours, the 4-blade will work very well, and you're not going so fast as to have a huge effect on top speed.

Here's a few links if you'd care to check it out:

www.comprop.com/

www.comprop.com/mercma.html

www.miwheel.com/propellers/outboard/

www.solas.com/newweb/propeller/search/mercury.asp?cat=D&SKindNo=S01&Skind=MARINER,MERCURY%20OUTBOARD%20@%20MERCRUISER%20STERN%20DRIVE

boatpropellers.iboats.com/propeller-showcase/solas/





Of course, if you've got $$$ burning a hole in your Wallet, your ol' Beast will really like a Quicksilever High 5 prop! I've seen some pretty good deals on Craigslist and eBay, you never know!

www.mercurymarine.com/propellers/elite/high-five/

HTH.............ed

p.s. note that you can use any "modern" prop on your Tower of Power, with the right thrust hub/spacer combination.