I was out this weekend on a longer trip and got a chance to get some better numbers with not too many people on board. At WOT i can get the RPMs up to just about 3600 RPM, about 600 RPM short of where i think it should be.

Based on the information I have gathered the 35' Sports Cruiser with the 350q (@235hp) should be using the 20x22 props. When i purchased the boat and pulled it out of the water I (luckily) snapped pictures of the props (which are I am now noticing are slightly different!?). They are 21x24:

If i'm looking at this correctly these are an inch to big and have 2 (inches?) too much pitch.  The pitch correction would give me about 400 RPM (get to about 4000 RPM theoretically).  

So here is question #1: 

*How much change in RPM can I expect if I drop down an inch in prop size?

Question #2: 

*Does anyone know the shaft size, tapering?, and hub length?

Question #3:

*I found a set on ebay that look correct but it has hard to tell condition, is a there a source for new props?

Question #4:

*Is it possible to have these replaced in the water? 

As always thank you for your valuable input! 

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Yup, that's way too much prop for your small blocks. You're correct in original prop size --- 20 x 22 x 1 3/8", T-36 three bladers.  Shaft data is found in our files section -----

S.A.E. STANDARD MARINE PROPELLER SHAFT END DIMENSIONS REVISED

Reducing diameter by 1" isn't gonna do it for you. You'll only gain about a hundred rpm or so. Yes, props can be changed in the water by an EXPERIENCED diver. The danger here is letting the key slide up on the keyway taper and getting a prop that is not centered on the shaft. Experienced divers are careful about this.

I would have these props pulled and taken to a GOOD prop shop and have an inch removed from the diameter and repitched to 22". The reason I emphasized good is that these are big props and pretty thick in the hub area. They are a bitch to repitch unless the shop has some really good presses. Most shops will just heat the hell out of them which affects the temper and strength of the props. Unless you know the seller and the used prop history, buying used props can be a real turkey shoot. As to new props, open wallet wide. I'll bet these props are well north of a grand each.

Thank you all - I found a shop that should be able to do the work and recently re-equipped to handle some very large props (http://www.thomasmarinepropeller.com/) . I had not thought to look into adjusting the existing ones, but will definitely look into this avenue now. Now to find some divers!

Another option would be to sell the current props to finance new ones. Might even be cheaper in the long run than having the current props rebuilt. And faster (less down time).

Update on the progress (aka bleeding of the wallet):

Props got pulled off today and will be at the shop tomorrow - should more details then.

Divers comments: 

-Port was easy enough

-Starboard took him over an hour to get off, the nut used was actually two nuts welded together, they used a stainless steel key that somehow lodged (says SS key is a no-no), prop also weighed more and had some kind of sleeve? Will know more once I hear from the prop guy.

they do make shims that go on the taper of the shaft to convert one diameter to another. In fresh water the metal difference does not make as much of a problem as in salt. But if the key gets bound up in the taper, it will wobble. Yes I've changed about 70-80 props under water and have encountered numerous problems. 1 nut and a cotter pin, in my opinion is better than a lock nut. My dad had lock nuts on his 1948, 46' Chris and lost a wheel while shifting to dock the boat. It took me three hours to find it on the river bottom. Dave    

Well i got the props back on friday and they got installed back on that same day. Pretty great service all around and I was very happy with the service. I went out for a quick spin to see how well the new props got me to my target of 4200RPM. 

Prior to the adjustment I was able to get to about 3600 RPM without having to do anything with the trim tabs. 

Now I can only get about 3200! However when I when I use the trim tabs I can get it to about 3800, I didn't leave it at WOT for long but i think i could maybe squeeze out 4000 if i let it work its way there (again with tabs). 

I'm going to take the dinghy off, its on a davit and i think its a huge wind catch that may be causing unneeded drag. Still strange to me that I was not able to get higher RPM without the assistance of the tabs. 

I will update once i get a clean run and see if that works. 

Did I miss what transmissions you have?

Paragon P34BL and BR respectively.

Well as has been said the 20x22 13/8 are correct. Your dingy won't make much difference on an 8 ton boat.

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