1970 15' Mirrocraft w/ Merc 50HP. Help me find my starting point for a proper prop.

[Roadblock]

OK so I'm pretty new to boating. I have a 1970 Mirrocraft Titan 15 foot with a Mercury 500 Thunderbolt 50HP.

I have an SE Sports whale-tail which gives me mixed results. If I don't run it, my prop vents a LOT and slips in the water. If I run it, it runs better but I think it slows the boat down a little. One time I GPS tracked the boat and was right around 28~30mph without the tail, but I've never checked with the tail on there.

If I'm out fishing and I go from point A to B, I don't really need the tail but if I want to pull tubes, I do and if I take it off and turn the boat almost drags to a stop and gets REALLY slow.

Motor as mentioned is a 1970 Merc 500, 13 spline. I have had 3 props but I broke one in the river, took a blade clean off. Was a factory Merc prop, 10 1/4, 13 spline, 13 pitch. I'm not even sure if it's the right prop because the factory service manual says it should be 10 3/4 inches.

Anyway I have two backups made by Michigan Wheel here locally, both are 10 1/4, 13 spline, one is 12 and one is 13 pitch. I run the 13, I've never ran the 12, it's still new in the box. Never saw a point to run it, my understanding is that it's slower.

I want to get more speed so so I can whip some tubes around. I know people with similar boats that haul at like 40~45mph. My motor is good, compression tested last year on all 4 at right around 140~145 -+ 2~3psi depending the cylinder. Carbs were cleaned this past winter, seems to run good.

My friend told me he thought my prop was too low of a pitch. Asked me what my RPM range was. I do not have a tach though. I know I'm going to have to get one to get this boat setup properly. Going to order a one next week when I get paid.

However, what starting point should I be at with a prop? My friend has a 16 foot Starcraft that sorta looks like mine with a 50HP Johnson and he is running a 17 pitch and gets 40~45mph. I do know his boat is lighter than mine, I have one of those foam filled hauls with no stringers, he has no foam and stringers.

I went threw my factory motor manual and found this section. From what I'm looking at based on boat length, weight etc, I probably want to be at a 17 pitch maybe? I don't know, maybe I don't understand working boat loads etc. Do I ignore the speeds listed and solely rely on tach readings once I get one installed?

Also here is the boat I have:

 

[steelespike]

I would say run the 13" pitch get the lightly loaded wot numbers.Rpm and gps speed.
Your gear ratio is 1.64 or 1.83. The Johnson is 2.42 So prop pitch will be different.
Neither boat is likely to see 40.A very rough figure if your merc. can make 5500 with the 13" prop
a 1.64 ratio would be about 36, 1.83 would be about 32. These are very sketchy speeds.
Get us your lightly loaded wot figures.We can figure things from there.

 

[Roadblock]

OK will do! Should have my tach here in the next week or so.

My buddy has a GPS speedo on his boat so he is sure of his speed but it's VERY light compared to by boat and he is also half my size!

 

[Sea Rider]

If having excesive prop aeration on straight water courses, choppy waters and horrrible ones at close turns, your starting point should be an engine/transom height optimization, once dialed can go from there all you want playing with prop pitches.

Happy Boating

 

[Roadblock]

Hmmm

Straight water courses, yes.
Choppy waters, yes.
Close turns, yes.

Unfortunately I don't think I can adjust the motor. It hands off the boat but without cutting down the transom, there is no way to lower it more than it is. Also the security bolts that go threw the transom and into the lower part of the motor really don't have much movement on them. I mean the bolts don't move but the they are not round holes, oblong ones that would maybe allow the motor to drop another quarter to half inch? Would have to pull the transom, drill out the holes, and refill them with West System resin and redrill in a lower position but the pain of it is most of the back end of the boat had to have the rivets drilled out to disassemble it and then they had to all be re-riveted.

I will post some picture as to why the motor might be hard to move and maybe you have some thoughts.

That said the plate on the motor, the fin is DAMN here level with the bottom of the back of the boat, that's how I thought it was supposed to be. If it's not 100% level, its maybe off by a half inch give or take which when I redid my transom, that might have been my fault because of the aluminum angles I used to make a transom cap, they were thicker than my factory transom cap.

 

[Roadblock]

OK here is a picture of before I replaced the transom. The plate on the motor is pretty much on with the bottom of the boat. if I changed that, it's only by maybe a quarter of an inch due to my new aluminum angles transom cap which I will post some pictures of.

Doesn't look like I changed the height that much.

 

[steelespike]

If venting is a problem try adding weight at the stern and or a good aggressive prop like a Turning Point Hustler.

 

[Sea Rider]

Congrats, very nice carpentry job, as you have experienced dialing AV plate even or sligh lower than boat's bottom or keel does not say a thing, you are a clear example of this issue that's cast on stone which I completely disagree by the large amount of proven examples that have been posting here showing the contrary. The issue with yoiur nice combo is that engine needs to lowered 1-2" who knows ? which is zillion time comsuming than shimming en engine up that any one could do. Chopping transom down are big words.

You have the 3 worst possible tech cond at same time, excesssive aeration at : straight, choppy, close turns, nada bueno, a prop change won't help much as expected as you have already less water bed amount under AV plate for prop to grip well that traduces in exccesive aeration and loss of best prop thrust, assume prop slip increment as well.

To have 3 tech issues at same time was the wot test conducted with engine trimmed to achieve boat running parallel on flat calm water cond, provided that deck was well balanced ? what is the wot rpm of that engine ?

Happy Boating

 

[Sea Rider]

To have 3 boating issues at same time, this is what's going on at back transom : Your engine has power or manual trim ?



Water flow is passing (red line ) very close to AV Plate, when should be passing skimming right under (green line) Water Deflector Plate. If you look back while at plane trimmed to have boat riding parallel to water level and provided that weight is evenly distributed on deck, will see upper exposed AV plate riding near out of water. Would it be possible to install a Jack Plate on transom and lower engine say 1-1 1/2" down ? if not, your last resource will be chopping middle transom down, just the width portion of the engine and re arranging transom plates to new height along remote cables. What's the distance from lower WDP to upper AV Plate ? seems very short, typical of older engines compared to actual newer ones.



Generally speaking when boaters achieves excessive prop aeration what is usually done is trimming engine down to compensate, probably not all 100% of aeration, but doing so will loss best prop thrust as the angle of attack will encounter water flow at speed at not the best prop angle to grip spot on..







Chopping transom down if jack plate is out of the question is entirely up to you. If having a complete carpentry along proper tools, would go straight for a chop down, but that's me.

Happy Boating