Cupped or not for F-90 Yamaha/17'

[Seahawk170]

My boat is a 17' Aluminum runabout style with an 86" beam, 14* deadrise, with empty hull weigh of 1175 lbs, and holds 34 gal. of fuel. Currently my boat has a 70 Yamaha 2-stroke, but recently bought a F90 as a replacement to be installed soon.

I researched similar sized boats w/F90 on the Yamaha website and concluded that the 3x13x19 prop is the closest match, but the 18' Duckworth (very similar to mine), shown a top RPM of 6150. I'd prefer that my max. RPM's with 2 people and gear not to exceed 6000, so will a cupped 13x19 slightly reduce it's max. RPM's?

The old 70hp with it's over-propped Yamaha 13.25x19 propellor gave me 30mph per GPS at 5000 rpms (max) w/2 people, gear and 6hp auxillary.
My old 70 weighs 228#, but my newer F90 tips the scale at 381#, so I'm trying to factor that as well, plus plan on getting a 4-stroke kicker later.

Any advice from similar circumstances is greatly appreciated.
Thanks.

 

[steelespike]

Your not likely to find an exact comparison.I think you have a perfect setup.Put the 19" prop on and see how you like it.
Take your time and see what it needs.
They say that adding cup is like increasing pitch one inch.But see if you can tell about stern lift or bow lift etc.

 

[Seahawk170]

Thanks steelespike, I will first test my F90 with my old motor's 19" pitch prop, mounted with it's anti-cavitation plate even with keel.
I did notice a bit of prop slip on my old 70 hp with anti cavitation plate mounted even with the keel, so if it acts similarly then will go with a cupped prop and my best guess on it's needed pitch size for RPM's. It appears my only choice may be the Michigan Vortex in aluminum to help with grab if it's slippage continues.

 

[steelespike]

Slippage? if you mean the prop loses grip that is called "ventilation" caused by air interfering with the prop.. It may be the prop or the motor is mounted too high or too low. Something disturbing the water in front of the prop.There are holes in the mounting bracket to adjust the height.
As a starting point the anti vent plate(just above the prop) should be about even with the bottom of the transom.It may end up a little higher or lower.
If necessary any number of props could solve venting issues.It's also possible to add some cup to a prop to both improve grip and improve boat attitude.

 

[Texasmark]

Most alum boats have a rib covering the "keel" area which is where most alum. boats (older Alumacraft excepted) have a welded seam....covers the seam and looks nice...something upon which to skid the boat, run down trailer rollers....... This rib is directly forward of the prop and especially on a turn, it generates turbulence (bubbles) and these bubbles get into the prop input water and help to cause ventilation. This is in addition to the ventilation caused by the prop being too close to the surface and problems associated with running a low performance prop.

As spike said, test it first then decide what to do. I like to run my engines mounted as high as they will run and still get satisfactory performance. Cupping on a prop helps the prop to grab water, not air, in these situations. With an additional 100# that far astern, you may notice more ventilation with the 90 than you have with the 70. Also with 20 more ponies, you will have more thrust increasing the likelihood of prop ventilation......when the prop turns it grabs what it can from the path of least resistance. If it's water, fine. But if air is more convenient, it grabs that. You may have to drop the engine a hole (?") and go with a cupped prop to get the performance you desire. The 14" dead rise will contribute to ventilation in turns over a flat bottom. If you add a kicker later on, you are just adding to the problem of ventilation with the prop close to the surface......you might just wait on your new prop purchase till you add your new kicker then look at your issues. Iboats sells lots of great props cheap. BTDT Also in the prop section is a tutorial that will help you in deciding on which attributes are right for you.

Final comment on cupping, if your speed is adequate to realize it's presence, cupping can appear to add 1" of pitch to your prop besides better grabbing quality which may increase your speed and lower your rpms accordingly.

 

[Seahawk170]

Thanks for the great explaination steelspike & Texasmark. In the past I always considered a slight freewheeling of the propellor as cavitation, but your nominclature sounds well described to me. I only noticed the slight sprop slippage a few times and it briefly happened during initial hard acceleration.
My boat does have a slight rounded keel so your explaination of disturbance from that makes perfect sense. I do remember asking about why the skin on bottom of my boat extends a couple inches beyond back of transom and then notched back in nearest the keel, and was informed it was designed for better prop performance.

Next Saturday I'm having an aluminum cap built/added to my transom and then will be installing my F90 with it's 25" shaft, so hopefully that will also move my prop back another inch into hopefully less disturbed water. I will definitely heed your advice and mount the motor with it's anti-ventilation plate even with the keel. For my first trial run with the new motor, I'll see how it performs with my old prop, and if slip is apparent then that Michigan Vortex? cupped prop they sell here will be quickly on order.

Thanks for the great explaination and educating me about ventilation issues.
Now I'll have to read up here on the differences between cavitation and ventilation, so any future questions will hopefully be a bit more tolerable.

 

[flyingscott]

Most of the new props have some extent of cupping from the factory. For a prop call the dealer yamaha has a guide that can tell what prop they recommend for your boat and you can go from there.

 

[Texasmark]

Keep in mind that when you move the engine back (referred to as offset) there is a number used on this forum to determine the height vs setback. Been too long and don't remember the names, but the number comes from the pros who do and did frequent the site. Reason being is that the boat is usually in a bow high attitude when running out and once it makes it's place in the water, the water attempts to immediately go back to where it was meaning that it's higher with respect to the bottom of the hull the farther back you go. So, to prevent the engine from being too deep in the water you have to raise it as you move it back. You can go to the archives and pursue this subject and get the number. I think it's 1" up for every 6" back, meaning the AV plate is 1" above the straight line extension of the bottom of the boat (minus the keel cap) for every 6" the transom clamp is aft of the normal mounting position in the transom.

Course you only need to worry about this if you are after every mph possible out of your rig. If you don't raise it it just means your LU is lower in the water generating more drag, but the bright side is it also ventilates less.

On Cavitation vs Ventilation, being raised on the Texas coast with no association with real professionals, the term for the plate above the propeller was "Anti-cavitation" plate. Joining this forum caused me to get whacked across the knuckles with a ruler and the difference in the two words was explained to me. Ventilation is the sucking of ambient air into the prop wash of the propeller thus reducing the thrust and that is why the plate is there.

 

[Seahawk170]

Thanks Texasmark for another great explaination. My transom raise is primarily just going up 5" plus the thickness of ~1/4" plate aluminum, so my estimate is that my prop will be set-back 1 1/4" to 1 1/2" than the original 20" transom height, (including the transom angle of ~15 degrees).
My little boat will primarily be used for fishing and crabbing so maximum speed is not an issue, and I'll be sure to take serious heed to your suggestions if I notice prop slip. Thanks again for the advice, and I wish you all the best for a great season on the water!!

 

[Seahawk170]

Most of the new props have some extent of cupping from the factory. For a prop call the dealer yamaha has a guide that can tell what prop they recommend for your boat and you can go from there.

Thanks scott, I already researched the propellors used on aluminum boats similar to mine at the Yamaha web site, and it definitely appears that the 3x13x19p prop is the closest one. If the old prop (13.25x19) causes issues, then I'll take your advice and contact my nearest Yamaha dealer as you suggest.
Thanks again for the good common sense advice, as there are many things I can easily overlook.

 

[Texasmark]

Thanks Texasmark for another great explaination. My transom raise is primarily just going up 5" plus the thickness of ~1/4" plate aluminum, so my estimate is that my prop will be set-back 1 1/4" to 1 1/2" than the original 20" transom height, (including the transom angle of ~15 degrees).
My little boat will primarily be used for fishing and crabbing so maximum speed is not an issue, and I'll be sure to take serious heed to your suggestions if I notice prop slip. Thanks again for the advice, and I wish you all the best for a great season on the water!!

I wouldn't worry about that setback. Take it out and run it and see what it does. Then if you aren't satisfied let's work on what you don't like.

 

[Seahawk170]

I wouldn't worry about that setback. Take it out and run it and see what it does. Then if you aren't satisfied let's work on what you don't like.

Thank you sir, your expertise in this subject is much appreciated.

 

[pckeen]

I have an 18' aluminum boat - with the same motor. We had a 17" prop, which gave us a great hole shot, but would over-rev the motor when we ran it flat out. We put on a 19" Solas prop, and it now tops out a about 5700/5800 RPMs flat out. That 19" prop should be fine.

 

[Seahawk170]

pckeen, thanks for the confirmation on the prop size. I hope that mine will have very similar results.

 

[Texasmark]

pckeen, thanks for the confirmation on the prop size. I hope that mine will have very similar results.

Don't be disappointed if not. Hull shape means everything. Case in point, my stock 90 running 48mph on a 17' boat. It's all about the hull drag. Notice in my avatar, that not much of the boat is in the water, as witnessed by the narrow wake and hull to water contact is drag and drag eats away at mph! The boat is unusual for an alum in that it has 3 distinct steps to the hull and at WOT it's only running on about 2' of the hull width.

 

[Seahawk170]

Texasmark, thanks for yet another good explaination!! While retrieving my boat from the fabricator this weekend I found out that my boat's hull was not specifically designed for a prop. The fabicator/machinist used to be a commercial guide on the Kenia in Alaska, and explained that the underside of my hull was specifically designed for a jet-drive outboard, but said that my prop. outboard would still work out okay.

The lower "V" of my boat has an extruded aluminum cap which stops about 5' ahead of the transom, and then aft the "V" is smooth. While looking down the underside I also noticed a slight taper along "V" of 1.5" to 2" (relief?), towards the transom. With my old 70hp my boat would run ~24mph at 4200 rpm's (2 people +gear), so I'll be very happy if I'll see a couple more mph with the 90hp pushing it. If my boat will top out around 38 to 40mph, then I'll be fully satisfied with it.
Next weekend I'll be pumping out around 25 gallons of pre-mix from my tank.. sure hope my neighbor will still loan me a few of his empty 5 gallon Jerry's. LOL
Thanks again for the insight, and I'll be sure to post my results from my repower.

 

[Texasmark]

We'll be waiting. Good luck