Is my prop the right size?

Sergi

Petty Officer 3rd Class
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Jun 12, 2012
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I just rebuilt my boat, it's a 1992 Sea Ray 200 bow rider, and by now we are making her first rides on the river.
The problem I have is that I expected that boat to be faster. I completely replaced wood structure with composites and currently still doesn't have interior trims and seats/cushions other than the captains and admirals.

Today we made a good testing session and those are the numbers:
25mph @3050rpm.
38mph WOT@4700rpm


At 4700rpm the engine seems to cut ignition intermittently as it had some rev limiter, but I thought those where the max sustainable revs as per manufacturer.
The boat hadn't any drivetrain when I bought it, so I rebuilt and installed a Volvo 4.3GX carbureted (205hp v6) and modified it's harness to work with my mercruiser boat. I also installed a rebuilt Alpha one drive. I confirmed yesterday that it's gear reduction it's 1.84:1. 18.4 engine turns for 10 at the prop.
This morning marked the prop hub and engine coupler for a slip test and this night the marks where still aligned.

I almost forgot, the propeller is a plain mercruiser alum. in perfect shape. 14x21p (by reference).

The fact is that I was waiting for something between 40 and 50mph when I rebuilt this boat for lightness. Aiming at the 50's.

The dry weight as per manufacturer it's 2,500lbs (but I assure that it's lighter) and the tests where made with two persons aboard and half tank. Almost full and three people I got the same results four days ago.

It's possible that the rev meter it's toast, and that would explain the engine cutting, but would leave me in worse numbers. I bought it 2nd hand, but it seemed ok.

Are those numbers the max. I should expect from that hull? It's the design?
 

Sergi

Petty Officer 3rd Class
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Jun 12, 2012
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Ah, I forgot to mention that the hole shot It's really nice and it seems not to bother for skiers. just 1mph loss, but those skiers nor me are bear like.
Another thing I observed it's that changing trim from side to side doesn't seem to have effect on the cruising. At full out I notice the exaggerated bow raising on start and today seemed to have a slight effect cruising on waves+wind, I had no success on modifying the cruising plane all other days.
It's my first boat, and call me stupid, but I can't see anything different on plane. Nor spray or anything. (always cruises like a champ)
 

SkiDad

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Jul 18, 2010
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yeah i would expect at least the low 40's out that setup. I wonder if you are going by the speedometer in the boat - I know mine reads 5 mph lower than it's actually going - for example 30 in my boat is really 35 - you can take car GPS with you or load a MPH app on your smartphone.

you could look at the same pitch in SS to drop your RPM a little and give you a few more mph.

here is a thread that might help.
http://clubsearay.com/archive/index.php/t-17197.html
 
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Chris1956

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Mar 25, 2004
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I had an '88 SR 18' Bowrider with a 4.3LX (205HP) Mercruiser. The low 40s was all I saw. With some tuning you should see near that. A prop with a bit more rake will give some better performance. Are you trimming the outdrive to achieve max speed?
 

Sergi

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@mitzysman: yes, those speeds are took from the GPS on my phone using a speed app. Sorry for my limited English Chris, but what do you mean with more rake? With trimming I didn't appreciate any difference in ride. Perhaps is the consequence of having a so lighter front hull on a calm river.
 

steelespike

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Apr 26, 2002
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Assuming your numbers are correct your slip is very high,25% 10 -13% is more typical.
Does the boat respond to up trim? Does it run on the rear 1/3 to 1/2 of the boat at wot(wide open throttle)
Does the boat lay down like it is glued to the water possibly even steering oddly at wot?
You may wan't to check your gps speed against your car speedometer.
All Tachs are suspect until we varify their results.If we assume your gps is right if we use 13% slip at 38 mph we get a rpm of about 4000.
You may be able to borrow a shop tach or buy an induction tach.
 

Chris1956

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Sergi, Your prop is a standard one. A high performance prop will have the blades "raked" back a bit. This may give better performance. As Steelspike says, you need to lift the bow by trimming the motor. If it won't lift, there could be other issues.
 

Sergi

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Jun 12, 2012
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Dear steel and Chris, thanks for your info. What I was trying to describe is precisely that, if I understood well, the boat seems to have it's static planning position, that seems a good one. But yesterday with rough water I was unable to lift the bow to cut the waves. It only seems to respond trim at very low speed, before planning. The speedo is dead, and the GPS app is accurate. I usually use it as HUD for my car (jeep Wj) The boat runs on it's rear half to one thirdth, if you mean the spray position. If it makes any difference, I installed an exhaust tube when I rebuilt the outdrive. But I saw all gases coming out through the prop at low speed. True that at high speeds I can see spraying like on water bikes, but much lower (1 mt) and gases around gimbal.
 

Sergi

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Yes, I put my head under the swimming platform at speed... Uh! The outdrive raises and lowers as it goes up to limit switch and downside forces the pump. You mean that could be possible the rams to be forced and the outdrive to stay at the same position at speed? I lost trim sender position three times, but I think it's the pin moving.
 

Chris1956

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Well, If you have spray coming off the back 1/3 of the boat, it sounds like it is trimming. Have you put a straight edge on the bottom of the hull? My SeaRay had a straight hull at the keel, until the very transom. The transom had a hook, to help her plane better. It might be interesting to see what you have.
 

Sergi

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The keel of my boat is completely straight. I don't perceived nothing like a hook. The lateral stakes seem to spoon slightly at the aft. Tomorrow will have another ride. Do you want me to test anything? In a moment I'll try to go and fix the loose gimbal pin.
 

Chris1956

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Sergi, I would recommend someone else drive and you observe the hull while underway. First, with trim all the way down, see where the hull meets the water in the front. Next, trim all the way up and see if the hull meeting the water moves sternward. If it does, maybe all you need is a better prop.
 

steelespike

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We need to confirm the tach,borrow a shop tach or use an induction tach.With your excellent prop it seems high slip(25%) is unlikely.
If we use 4700 at 10% we get 45.7 mph,13% gets 44.7.
So this indicates the tach may be off high.
From your indications If the tach is accurate then your "excellent"prop may be suspect.
Is there a hydrofoil on the outdrive?
 
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Sergi

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Jun 12, 2012
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Sergi, I would recommend someone else drive and you observe the hull while underway. First, with trim all the way down, see where the hull meets the water in the front. Next, trim all the way up and see if the hull meeting the water moves sternward. If it does, maybe all you need is a better prop.

This past Saturday I had the opportunity to do that test. With the drive trimmed inward, and at the cruising speed (25mph), the water contacts the front of the hull. With the drive trimmed outwards the water contact goes to the mid-section of the hull. Even the prop starts to cavitate.
I saw the contact position from mid to rear, but I must have been wrong. I thought that for correct planning the boat should be less in contact with water. With spray from mid line to aft.

Waiting for the videos taken by spotter.

Anyway, I managed to lock the starboard gimbal pin and the tilt reading on the gauge is pretty accurate. The water was so calm and I noticed 1mph gain when trimmed up during one ski pull. So does have effect.
Perhaps someone could post me a tilt position on cruising at 25mph with some 20ft hull for reference. Does it have to be more on the upper half of the dial or the lower? Just asking for experience common sense. I'm extremely novice.
It seems that I like best to cruise at 25% of total tilt. Normally calm river water.

My setup does not have hydrofoil.
 
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steelespike

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Do we have a wot rpm and gps speed from a lightly loaded test run?
You need to trim up at wot to the best rpm and speed.
Your best practical cruising speed with best mileage will be somewhere from just on plane to about 1000 rpm higher.
You would normally trim down to help maintain plane at reduced throttle. This is a trials and error procedure or you could get a flow meter
Some do real time mpg calculations others just fuel flow. Your looking for the best speed at the least flow.
Don't assume that your slowest planing speed is best mileage.
 

Sergi

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Jun 12, 2012
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1. Yes, all the numbers where 2 people + half tank. That in a boat that only has main seats, no others already present (nor windshield).
2. From half up, I understand.
3. Plane at almost 3000 so 3k-4k.
4. Flow meter? Will investigate that.

Understand. The fact is that I found out that the boat does a amazing mileage. So I suspect that this rev limiter effect may be fuel surge. Paired with some accesory bearing failing that makes a pretty high pitch sound at high revs and a rev meter that shows more could be my problems.
I realized that I can record a video the next time we have a go so you can appreciate what the meter is showing and the sound from engine.

I always had I6's so I don't really know how this v6 stuff sound like. I feel over revving.
 
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