Porpoising - would a 19 or a 26 pitch prop be better?

Hi all,
I have a saltwater 1970's 21ft Mako, with single Mercury 225HP four stroke. As there is a fair amount of weight on the back it initially porpoised a fair bit. I added a wedge infront of the outboard which helped with reducing porpoising greatly, however I have read that by changing the prop (I.e. Lowering the pitch, say from my current 26 to say a 19?) could also help??

I have also read about adding Trim tabs (seems to be frowned upon for small boats) or adding a 'whale tail'.

I may also move the battery towards the bow to help with weight distribution also.

Has anyone got any advice on what may be the right route to go? Specially, do you think that moving from a 26 to a 19 pitch would be worth a try?

Really appreciate any advice.

 

If you put a 19 pitch the speed will drop and the engine will over-rev. A prop with some rake will give some lift and bring the bow down. Trim tabs are a really good way of adjusting trim. How are you trimming the motor? That makes a huge difference in porpoising?

 

Nothing wrong with fitting tabs to the boat, the extra weight of the 4-stroke might be the problem.

 

Re=distributing weight may help, but you should look at trim of the outboard first, not only the in/out trim but it's height on the transom. I would not recommend moving the battery. If you do you will probably have to go to much larger cables because of the much longer length of the cables. Take a look at where you are storing things, Where your fuel tanks are, etc if you are looking at moving weights. But I would agree with Gonzo said, trim tabs would do the job and are common even on small boats.

 

I'm with Peter and suspect the ventilation plate is too low and/or the leg trim needs to be looked at. You can change props, but if you decrease diameter, you have to increase pitch and likely cup and rake too, just to keep your engine in it's operating range.

It's very probable you have a combination of issues you're battling, including a saturated floatation foam block that may be altering the boat's trim, which would be typical of a boat of that era and/or structural problems, like a trailer hook or rotten transom core.

You can hunt and peck at it, or address the individual areas, making single changes to see their affect on the boat's handling.

Assuming the transom core checks solid, the boat doesn't have a hook or other structural issues, like rotten stringers, I'd start with the ventilation plate's height, which should be at the bottom of the boat, not lower. Start with it dead level with a straight edge off the centerline of the boat. Next would be trim adjustments to get her ride angle right (about 3 degrees positive). If you have tabs or shingles, cut them off and start with a level playing field and check to see if the boat has developed a hook.

Lastly, consider if you're over powering the hull, for it's shape and loading. I'll assume your "1970's Mako is the 1977 - 1983 Angler 21, which is only rated for a 200 HP, 450 pound 2 stroke outboard. Your 225 Mercury is 10% over the maximum rating and in the 650 pound range, which is 30% heavier. Both of these situations may be the cause of your issues and though moving around some weights might help, you're likely just asking too much from a nearly 4 decade old hull. For example, what's the condition of your transom core?