Prop/Speed Question

[Baslan]

I have a 15'3" 1996 Seahunt CC with a 2002 4-stroke 50hp Yamaha. The carbs have been rebuilt, sparkplugs changed, fuel lines changed, new fuel tank, all within the last three months. My question is this: how fast should I expect this boat to run? I have no RPM gauge so I can only assume from the sound but when I'm all out I do 24mph alone and 23mph with 3 people on board. I seem to cruise at about20mph either way. This seems odd to me. Shouldn't this go faster when I'm alone? I seem to have no trouble planing regardless of the weight onboard. I do have fins on the engine. Could it be that I have the wrong prop? What should be the prop on this setup? Thanks

 

[steelespike]

Re: Prop/Speed Question

Welcome,Yes you should be faster.
you need to read "read this first"at the top of the page.
Get us as much info as possible.
Prop size should appear at the base of the blades or under the prop nut.
To do this right and for the health of the motor you will need a tach.
A Yamaha tach, generic brand or something like a TinyTach.
In the mean time take off the fins for now.
The anti vent plate(where the fins were attached)should be even with or slightly above the bottom.
of the transom.Measure with a 3 or 4 ft straight edge along the keel to the plate.
Does the boat respond to up trim? Trim down for hole shot and trim up for best speed as the boat planes.
As you trim up the bow should come up and speed increase.

 

[Baslan]

Re: Prop/Speed Question

Thank you very much for the quick reply. I'm sorry, I should have read that sticky first.

The prop is a new Yamaha aluminum 10-5/8x12 that was recommended to me by a local mechanic. The prop that came on the boat was severely bent in several places and was clearly too large for the engine (wouldn't even plane). This new prop has me planing very quickly but doesnt get me to the speeds I feel this boat should achieve, as noted in my first post.
I'm not by the boat so I can't check how the anti vent plate lines up this boat/engine came with the fins so if I take them off, there will be holes where they attached. Is that a problem?
As far as response to trim, the boat is very responsive to trim and I achieve plane very quickly doing exactly as you have described. I find however that once I'm on a plane, if the trim is off even a little bit, the fins cause inordinate amount of drag and slow me down noticeable. The speeds I mentioned in the first post are with ideal trim settings, with the fins riding level.
While I grew up on boats and have a lot of experience driving them, the mechanical aspect can somewhat elude me. I mention this because I tried to install a generic tach that I was told would work but no matter what I tried, I could not get it to read a signal from the engine. I know that this engine originally came with a digital Yamaha multi-gauge. But shouldn't I still be able to hook up a generic tach? Is there a cheaper generic tach brand that you know will work this engine?
As soon as I get home I will post check how the anti vent plate lines up and this weekend I will try to find time to run the boat without the fins if you think those holes won't be an issue. If it seems the engine might be misaligned, I will post pictures.
Finally, for what its worth, the boat has bottom paint. Don't know if that would affect performance this much though.

 

[Baslan]

Re: Prop/Speed Question

I hadnt ever heard of TinyTach before, but after your post, I took a look and that might be a cheap alternative for me. Do you think the TT2A - TINY TACH (http://www.tinytach.com/tinytach/gasoline.php) model might work for me? Because I would be thrilled if I could get accurate rpm readings for only $47 and minimal installation. Thank you again for all the help.

 

[steelespike]

Re: Prop/Speed Question

I don't have any first hand info to offer.But as I understand it the tach will work on just about any motor with a spark ignition.
Some applications ie: motors with wasted spark ignition may require some math.
For instance on some 4 strokes the plug may fire everytime the piston is at the top of the cylinder.
So you may have to devide by 2. Its possible the 2 stroke setting may work on those 4 strokes.
Maybe you can talk with them to further understand.
Be sure you understand about the antenna wire and indicate number if cylinders,length of coax etc.
You may find more info here in the electric/electronics forum.

 

[Baslan]

Re: Prop/Speed Question

I saw several youtube videos with this thing being used on my exact motor So I took a shot and ordered one we will see. Does the propeller size I mentioned above seem like an accurate propeller for my engine in this application? Thanks again.

 

[steelespike]

Re: Prop/Speed Question

Is your speed by GPS?
Prop calculator with my guess of 6% slip your rpm is about 4200 should be around 6000.
Prop size is probably right, you really can't make up 1800 rpm with a prop anyway.The tach will help us
figure out how well your setup works.

 

[Baslan]

Re: Prop/Speed Question

I am monitoring speed via GPS, yes. I checked how the antivent plate lines up and it appears to be about 2" higher than.the bottom of the hull. If iI remove the fins are the holes left behind going to cause a problem? I could close up the holes with a bolt and washer is that going to produce inadvertent drag?
The TinyTach should be in early next week. When I get some RPM readings I will update the thread.

 

[steelespike]

Re: Prop/Speed Question

Holes shouldn't be a problem.It will be slower to plane without the fins.But we should be able to solve atleast some of that without the fins. I'm surprise at the 2" height.Are you measuring the plate just above the prop?

 

[Bifflefan]

Re: Prop/Speed Question

Those holes will not affect anything, unless they go into a place they can leak oil (not likely). You can put a piece of tape on the bottom and fill them with some epoxy, then paint it and you will never know they were there if they bother you.

 

[Baslan]

Re: Prop/Speed Question

I did a speed test today with the fins off. It definitely took longer to plane, about 6-7 seconds longer. I was sitting behind the console and another 200lb person in the front seat in front of the console. Once on a plane and trimmed all out was about 25mph. 3/4 throttle cruise was about 21mph. So a speed increase of about 1mph on each without fins. However, the stern was riding noticeable lower in the water without the fins. Seems that that should slow the boat down yet I gained speed, albeit marginally. This boat is used primarily for backwater shallow fishing and I think I would prefer the lift the fins give me for a shallower draft on a plane. With the fins I ride a very level plane and draft several inches less.
As far as the alignment of the antivent plate, I'm using a straight edge from the bottom on the transom. The plate sits about 2"higher above the bottom on the transom. I think the previous owner rigged this for very shallow access because I can see the old marks on thethe transom where the engine used to sit, a notch lower.
I wonder if the engine is sitting to high for optimal speed? But how much could I gain by lowering the engine an inch or two? Sometime next week I will receive my Tiny Tach and be able to provide more detailed information about the engines performance.
One more question: this has very poorly applied( looks like orange peel) very thick bottom paint. How much will this affect performance? I have absolutely no need for bottom paint.
Thanks to everyone for all the help. I really appreciate it and will greatly value this newly found forum.

 

[steelespike]

Re: Prop/Speed Question

I've noticed that boats setup for shallow water do seem to run slower than similar size and weight boats.
At 2" above ther bottom you may have a lot of slip.The Tach will tell us. A poor paint job could slow you some
If you don't need it wouldn't hurt to remove it.There are a couple of other ways to improve hole shot.
Move weight forward batteries,gas tanks anchors etc. May not need to go to the bow but to the mid area of the boat
where its easier for the boat ends to move around the weight concentrated near the middle.4 blade props usually improve hole shot and improve stern lift.Smart tabs may work better than the fins.It is claimed when carefully installed and adjusted they can improve hole shot planing angle and still maintain speed.
I wonder if the po may have had the prop doctored to improve its grip in that setup.

 

[Baslan]

Re: Prop/Speed Question

Regarding weight distribution, when I first acquired this boat, I immediately noticed that the stern was way too loaded. A 12-gallon gas tank, a heavy-duty battery, and a livewell situated near the stern, not to mention the driver, when behind the console sits only about 2ft in front of the transom. I moved the battery to the middle of the boat, it is now housed in the center console which is situated pretty close to the center of the boat. I replaced the 12-gallon tank (it was old and cracked anyways) with a 7.5-gallon tank to reduce weight. I also house the anchor in the bow compartment. I suffer from no porpoising or anything. I think I've achieved as good of a weight distribution as I can get without intentionally weighing down the boat further.
One thing I didn't do yesterday when I water tested was I didn't pay attention to where the antivent plate was riding in the water when on a plane without the fins. I know with the fins at optimal trim, they ride right on top of the water. But I think without the fins and the reduced lift at the stern, the antivent plate had to have been riding a good bit below the surface of the water because I definitely noticed that the stern itself was riding lower, but I didn't check the plate like a fool. But that would mean I would need to lift the engine even higher than it is for optimal setup. All this is hurting my brain, lol. I realize its all speculation until I know what RPMs the engine is running at to determine the prop slip so I need to quit broding over this until I get the TinyTach. Its just frustrating because I know the engine is running very well and the hull design is a shallow draft, minimal v-hull design that should be achieving greater speeds and efficiency with this engine.
I'm going to post a picture when I get home tonight of how the engine lines up on the transom so you guys can see. Maybe there's something there you all will notice that my ignorance veils.

EDIT: TinyTach just arrived. I'm going to try to get the boat on the water within the next 2-3 days and I will report engine RPMs at that time.

 

[Baslan]

Re: Prop/Speed Question

Took it out with the Tach today finally. At WOT I'm running at 4400rpm with typical load for a two man fishing trip, going 22mph. Based on a slip calculator, I'm looking at 25% prop slip. I put the fins back on because I like the lift so I know I'm losing a couple MPH there, but it still seems to me like there's something holding this boat back. Its got to either be the engine setup or the shoddy, jagged bottom paint, or a combo of the two I suppose, right? Throttle seems to he working fine, engine is fine, prop was recommended by a Yamaha tech so I can't help but think its physics and drag that's holding me back rather than any mechanical problems.

I put the fins back on because I was in the shallows today but I'd he willing to keep them off the boat if I could get the stern to have similar lift. But the difference with and without the fins is stark. If i take them off again, and if all this slip is happening because the engine is set too high, would lowering the engine somehow give me increased lift at the stern while increasing my efficiency?
I've appreciated all the help so far, and any insight into my enigma would be welcome. Thanks.

 

[Baslan]

Re: Prop/Speed Question

When I calculated my prop slip, I used 13 pitch on accident when I have 12 pitch, so my prop slip is more like 19%. Still seems excessive. Any suggestions would be appreciated. Would a 4-blade give me increased efficiency and enough lift in the stern to remove the fins? Thanks again.

 

[steelespike]

Re: Prop/Speed Question

I believe your motor is rated for 6000 at 4400 your 1600 rpm low.You can't make that up with a prop.
Does the boat get up on the water at speed or does it lay down flat like its struck to the water?
I think you should be able to make 29 to 32 mph.
Generally the slower a prop turns the higher the slip probably accounting for your higher slip number.
Paint job could be slowing you but I wonder if there are other issues. motor, water in the foam under the floor,
Throttle not opening all the way?

 

[Baslan]

Re: Prop/Speed Question

When I'm on a plane, everything feels fine in terms of how the boat is riding. It seems to be pretty level, but I don't feel like its stuck to the water, riding low, if that's what you mean. It responds to trim really well and everything.
I've checked the throttle and it definitely opens up all the way. When flushing the engine on land, I can easily rev up to 5000-6000 rpm so I know that's not an issue. The motor got a clean bill of health about 4 months ago when all the carbs were rebuilt, sparkplugs changed, and a bevy of other maintenance was done.

You bring up an interesting point regarding water in the foam under the floor however because I checked this morning and there is about 1"-2" of water sitting in the hull between the areas of foam that I can see. This got me thinking- is this why when I take the fins off the engine my stern rides so deep in the water? Because when I'm holing all that water in the hull rushes to the back and doesn't let the boat level out. I also noticed another alarming thing that I hadn't noticed before- at the bottom of my V, where the hull makes contact with the sand when I beach it, I noticed there's no paint left, no gelcoat left, and you can see the glass fibers. About a 1/2" by 1' strip of bare fiberglass basically. All seems sturdy still but I know that water can seep through exposed fiberglass, right? Could this be how my hull has slowly been filling up?
But its only a 15'3" boat, how much water could really be in there? Enough to weigh it do so much as to lose this much speed? I wish I could weigh the damn thing and find out how much over factory weight it is.

EDIT: I've done some research on this matter and apparently something as small as a 13' whaler can hold 700lbs of water if all the foam is soaked, so this could be some very bad news for me. When I get home tonight I'm going to take a core sample of the foam and see just how bad it is. In the meantime, I will ShopVac out all the standing water in there. If it becomes apparent that the foam is soaked, I think I've more than likely found the source of my performance problems, since this would easily be 100's of pounds of added weight. If that is the case, I will start a new thread in the proper forum. But any insight into experience you might have regarding water in small hulls would be appreciated.

 

[steelespike]

Re: Prop/Speed Question

Usually because a boat is stored bow up to help it drain if water gets in.Sometimes with improper storage snow/water and ice sits for long periods gradually being absorbed into the foam and sometimes into the structual members of the boat usually concentrated near the stern.It appears with your good response to trim that the hull form is good.Hopefully your water issue is just loose water flowing to the stern.

 

[AquaMojo]

Re: Prop/Speed Question

what is wrong if the propeller on an inboard/outboard motor does not spin?
the boat is a 86 searay 6 cylinder 4.3 GM engine