Propeller advice

[4n6fisher]

Quick question about correct propellers as I will be needing a new one after an unnamed (wasn't me) person ran into some rock in the San Juan's last weekend. Luckily it was just bent a little and did not derail the boat camping trip, and we turned just fast enough to avoid running aground, but not fast enough to eliminate the prop damage.
The boat is a 19.5' 1978 Glasply cuddy with the original mercruiser out-drive (pre alpha I believe). The only label is a -21 and seems to be about 16" in diameter. Anyone have thoughts if there is a better propeller pitch/size or should I just replace with the same.
Thanks fro any tips!
Omar

[18ftTrophy]

the -21 makes me think it's a 21" pitch. You might ask this question on Glasply.org to get a better answer. My question would be if your WOT put your motor in the preferred RPM range. If it did you could take your prop to the Prop Shop in North Lynnwood? south Everett for repair and it will look like new when you get it back.

[Sideburns]

Its difficult to make prop changes when the only one you have to test with is damaged.

To calculate prop pitch or make any other prop changes with an online prop pitch calculator, you must have a decent prop, an accurate top speed measurement and a tachometer to measure the rpm of the engine at top speed. (you'll need to know what the max rpm rating is for your engine too)

Some prop shops will sell you a prop, then let you exchange for another after you have tested it out and return with those measurements.

The only way to get a useful prop pitch suggestion from someone else would be if they had the exact same hull, engine, weight........

21 seems like a common pitch for a setup like yours. If your interested in better pulling power (lower number pitch) or higher top speed (higher pitch number) you could try changing to a 19 or 23 pitch...... BUT you'll have to be careful to not over speed your engine rpm with a 19 (must have a tachometer), and if your 21 was already perfect, swapping to a faster 23 pitch will actually slow you down! (*your tachometer will show a lower rpm at max speed)


Hope that's clear as mud. If I've lost you here, you should probably just order another 21!

ps. Dont go any bigger in diameter! Looks like you just barely clear the outdrive with the tip of the prop in that picture. The last alpha 1 outdrive I played with was on a smaller ski boat with a v6 and was 14.25 diameter x 19 pitch.

[4n6fisher]

Thanks for the tips, I am usually pulling a dingy along so I do not have much WOT running time to know. Novice boater too so no idea what the ideal rpm's would even be! Typically I can only go about 13-14 mph to keep the dingy from freaking out at about 2400 rpm's.
At least we did not end up like this guy! Ran his 45' trawler into a pinnacle leaving Ewing Bay on Sucia Island. Not the best picture looking into the sun but he was listing pretty good, luckily better help than me arrived rather quickly, think they waited for high tide to get off.

[4n6fisher]

Whoops that was the wrong image!

[4n6fisher]

Sideburns,
Thanks for chiming in with some sound advice. Guess I should stick with as similar as I can for now until I can get some on water WOT testing. I did like one four blade prop that stated better turning (hole shot) and better stern lift, which would be good in my case since I am often pulling a dingy and just barely fast enough to get on plane. Sounded like you loose a little top end speed with a 4 blade, but that is not my concern.

[hewesfisher]

A good prop shop would be able to ID your prop, repair it, and give you solid recommendations for your specific application.

[Sideburns]

A good prop shop would be able to ID your prop, repair it, and give you solid recommendations for your specific application.

Yup, thats a great reason to spend locally instead of trying to save a buck and order everything on amazon. (guilty)

[4n6fisher]

Heading to the Prop shop in Mukilteo, also found a thread on Glasply.org (nice recommendation 18ftTrophy) where a guy with the same boat was asking same thing so got some good tips on a 4 blade replacement. Might see if they can just repair this one to keep as a spare, depending on how I like the 4 blade. Since it was mentioned on a previous thread always good to have a spare on board, in case next rock encounter is worse! Yes Amazon is so easy since it eliminates leaving the property, but always good to get real human advice too and keep the local economy going and support the little guys!
Thanks for the tips everyone always great to get advice from more experienced boaters.

[Sideburns]

I dont tend to think of 4 blades just because the majority of my experience is with two stroke outboards. Supposedly they are somewhat less efficient too. But your larger, heavier, inboard/outboard 4 stroke is exactly where a 4 blade prop could excel, giving you a faster launch to planing speed and still giving you an acceptable top speed. Good luck.

[hewesfisher]

Yup, thats a great reason to spend locally instead of trying to save a buck and order everything on amazon. (guilty)

Not to mention save $$$ too, don't ask me how much I've spent buying props trying to improve certain performance conditions with my boat.

[4n6fisher]

Prop shop in Mukilteo hooked us up, great people and very knowledgeable. Went with the 4 blade, and they thought that would help performance with my boat. Now just have to get some time for a water (fishing) test. Also they were cheaper than Amazon!

[hewesfisher]

Prop shop in Mukilteo hooked us up, great people and very knowledgeable. Went with the 4 blade, and they thought that would help performance with my boat. Now just have to get some time for a water (fishing) test. Also they were cheaper than Amazon!

[18ftTrophy]

The Pro shop has done me well, let us know how you like your results.

[Sideburns]

Way to go Pro Shop! Thanks for the update.