Aluminum or stainless

[jsaylor]

1750 procomp bass ski boat, 150 1987 150 mariner, WOT 5k, trophy 22 vented, 6 inch jack plate 5 inches to pad. This is a real slug getting on plane, I don't care if I lose a little on top end, I was thinking a 19 or 20. My question is would I really notice much difference between a aluminum and a ss?

 

[Robbabob]

First question, is the WOT of 5K what you get or what the engine is rated to achieve?

Generally speaking, the aluminum will flex, which hurts your hole shot; so the SS will be better. The SS will not give way if your prop hits an immovable object. the result could be disastrous, but not always.

 

[Texasmark]

Gear ratio for later model basic 150 Mariners shows to be 2:1. I ran a 19P on a 115 I6 with a 2:1 also on a similar sized Ranger pad boat, but no setback and no jackplate. The prop was a Laser SS and was "vented" also. I had a good hole shot with the vents and nice top end. Naturally when you loaded up the boat with folks it suffered.

Based upon what I see as similarities with HP within 30% you would do well to drop the pitch and which pitch I would say depends upon how much load you usually carry. If you have 3-4 folks on an outing and pull a couple of skiers then I would opt for the 19.

SS usually are more efficient props depending upon the make/model. I changed over back in 1973 and having changed boats numerous times over the years I always ran SS. If you boat around rocks then I would not recommend SS. Otherwise it's the prop of choice for me.

Part of your hole problem could be the setback. Coming up on plane with the bow up the engine is pretty deep in the water meaning drag is high and at that angle the load on the engine is pretty severe. Larger ports could help to get on plane faster. Port size is a trial and error thing on your particular rig. To small and they don't do much for you. Too large and you over rev too much slowing the hole shot like you would have without them.

On my boat the ports were about 3/8" allowing for about a 1000 rpm over rev in the hole which lasted until I got up and running whereby the holes sealed off by water pressure and the rpms dropped as the speed increased with the prop locked in. I called it overdrive. Like the old days with a stick shift car and overdrive.

HTH,
Mark

 

[jsaylor]

Max RPM with my trophy 22 with vents (all open) is 49 to 5 k, I ordered a turning point 4 blade 19 aluminum, I'll post how this prop works out. What do you think of my jack plate height at 5 inches? (center of prop to pad)? motor has to be tucked down all the way until about 30 mph otherwise it will porpoise. Thanks for all your help

 

[jsaylor]

Just wondering if increasing the size of the vents in the prop would help, they are currently 1/4 inch, perhaps open to 3/8?, I don't notice the venting doing anything until on plain about 2500 RPM then I can hear and feel the change

 

[Outsider]

I don't notice the venting doing anything until on plain about 2500 RPM then I can hear and feel the change

That's the range vents cease to affect anything, water flow essentially seals the holes from exhaust ...

 

[Texasmark]

Just wondering if increasing the size of the vents in the prop would help, they are currently 1/4 inch, perhaps open to 3/8?, I don't notice the venting doing anything until on plain about 2500 RPM then I can hear and feel the change

"Feeling the change" tells you that they are working for you. Question is, are they big enough? 3/8 to me is not all that big as stated above. 2500 is where you'd expect them to seal up......with corresponding boat speed. I doubt holes will do anything for your porpoising. They will help your hole shot by allowing your engine to rev up higher developing the hp that turns the prop sooner.

Obviously when the engine is set back from the transom, different forces are at work. Controlling porpoising is a balance between hull vs thrust angles and speed. Whether or not your 5" of jacking is correct or not depends on distance of setback for one, per the old salts on here; past and present contributors. There is a rule of thumb and it is probably in the archives.....like 1" of height increase for every "X" inches of setback.

Obviously setback and jacking are part of the balancing act. On my boat(s) if I get/got into a porpoising situation I would do one of two things: Increase speed or tuck in the trim. If you are coming out of the hole and have to get to 30 to get it to stop then possibly larger holes would help the engine to get up to speed faster and help you to get to 30 faster. I have found that higher engine heights tend to reduce the effect of trim position on boat performance. Getting the AV plate out of the water forces the prop thrust to determine the attitude, not the AV plate, aka trim angle dependence.

The fact that you have to remain tucked all the way in just says that something needs to change. Since you have the jack plate, my first suggestion is to up the height 1" and come back with the results.

Have you tried the hole shot with the engine vertical or trimmed out slightly? I do this quite often and the boat does porpoise but it's a different kind. Frequency of oscillations are farther apart and only about 3. Each time you go through a cycle, the prop vents and the engine revs somewhat. That higher hp due to the rpm increase and faster turning prop bite as the bow comes back up and "rockets" the boat forward. By the end of the 3rd oscillation, occurring in a matter of a few seconds from dead in the hole, I'm at 30-35 mph and around 4000 rpm, holes sealed. BAM prop slip calculator verified my numbers with my assumed 12% slip.

Lot of jabbering here. Maybe it will help, maybe not. Let's keep the thread alive and keep us posted.

Mark

 

[jsaylor]

Wow good info, I'll try raising the engine first

 

[walleyehed]

I would assume you have a water pressure gauge??

 

[walleyehed]

EDIT: Actually aluminum flexes and goes lower in pitch with load on the prop...most alums will have better hole-shot than SS in the same pitch.

 

[jsaylor]

Results on the new hustler 19 four blade, does seem to pull out of the hole better, still can't get above 5k max RPM, lost about 4 MPH top end. Most likely will keep as a spare. gone from 22 trophy 5k max rpm, 55 mph to 4 blade aluminum 19 5k RPM about 50 MPH, the 19 4 blade I can exchange for a 17, should I exchange?

 

[Bob_VT]

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