Help me understand

[scanman]

I have a 1975 evinrude 9.9. After a complete overhaul, a 15hp carb, & Boysen double reeds, it makes around 18hp. It's mounted on my 13'6" alum. Jon. With me, the 12gal fuel tank, & the troller & batt, I'm gettin 21.9 mph on the gps. I've heard I should be gettin more, though. I've done moderate research, on this forum, & it seems I'm gettin more confused. Is there an easy way to understand the relationships between mounting height, tilt, trim, & prop sixe & pitch. I'm quite sure all these things are affected by each other, but how? BTW, I'm still runnin the OEM alum prop for the 9.9hp Evinrude & it's mounted al the way down on the transom as low as it'll go. The trim pin is in thesecond hole up from the transom, making the motor nearly 90 degrees to the surface when it's in the water. When it's on a plane, it does feel like it's plowin ever so slightly. It seems to have a bow down attitude, & never really gets to "skippin" over the water.

 

[JB]

Re: Help me understand

Some things about fine tuning the rigging can only be done by trial and error, scanman. Where to set the fixed trim and what prop to use are a couple. Outboard height on the transom is another. They do affect one another, so fine tuning the rig of a modified, fixed trim rig is more art than science.

Here are what I suggest as starting points:
*Set the outboard height so that the antiventilation plate is about 1" higher than the bottom of the boat.
*Set the trim pin 1 hole out from where the AV plate is parallel with the boat bottom. This might slow the hole shot a little, but it will greatly improve top end.
* You may increase height or tilt looking for improved top end, but do not allow the prop to ventilate or the boat to porpoise. Either will indicate too much.
* It is hard to predict what rpm your hotrod is going to peak at, but I would guess 5.5Krpm. Select a prop that will put you at 5.5Krpm wide open. Yes, you must use a tachometeer for this one.

Good luck, and let us know what you get.

 

[scanman]

Re: Help me understand

Thanks JB! I just took a few measurements & I'll have to buy or make a jackplate to get the motor up that high! It's about 2.5" below the bottom! I'll start lookin into that right now! As for rpm... I have a tach on it, & with the timing set using the "static" method ( with a multimeter ) it ran as high as 5750 at WOT. It'll get as high as 6500, but I'm sure it'll last a lot longer if I don't run it that hard. I was kinda thinkin the SS version of the stock 15hp prop would be a good choice, but I know nothin about prop selection, yet. Does that sound like it might be anywhere close to right?

 

[Dhadley]

Re: Help me understand

Don't count on it being 18 hp. Just putting in new pistons, carb and reeds will not increase your power by almost 100%. Yes, your motor is low which creates drag but your hull design will be the major factor in performance.

 

[scanman]

Re: Help me understand

It's a flat bottom with only 5 "ribs" runnin bow to stern. It's only 32" wide on the bottom.

 

[Dhadley]

Re: Help me understand

A flat bottom has lots of drag. How much hook?

 

[99yam40]

Re: Help me understand

It's a flat bottom with only 5 "ribs" runnin bow to stern. It's only 32" wide on the bottom.

Sounds more like a canoe than a boat. I would think that the more air and fuel in also needs more out. Did you change the exhaust any?
What prop you running right now? the old 9.9 or a 15hp one. more hp needs to be transferred to the water with a bigger prop

 

[steelespike]

Re: Help me understand

If you can raise the motor 2 1/2 to 3 1/2" I wouldn't be surprised to see 2 to 3 mph more.
A jack plate would allow fine tuning the height.A 9.9 is good for about 21 on a light reasonably fast boat.a true 15 about 25 a true 18 about 27.I also doubt you have doubled your hp.
While a jon will get up fairly easily;they can be difficult to keep the bow up.Also at only 32 wide
it may be difficult to go fast with the weight.You probably have 3 keels and lots of rivets,that doesn't help speed.
Forgot to mention the 9.9 is rated at 5000 to 6000 rpm the 15 5,500 to 7,000.
With only 5 ribs the bottom is likely waving like a flag.

 

[Bob_VT]

Re: Help me understand

Tashasdaddy (RIP) mad this bracket for one of his boats to raise the motor up

 

[seahorse5]

Re: Help me understand

I have a 1975 evinrude 9.9. After a complete overhaul, a 15hp carb, & Boysen double reeds, it makes around 18hp..

I really doubt that you increased the hp by 80% with just a carb and reed change. The 15hp has a different cylinderhead than the 9.9 does, was that replaced also?

My guess is that your motor is closer to 12-14 hp with just the carb change and reeds.

A 9.9 will push a 14' boat around 20-23mph and a 15 will do just a few mph better.

 

[5150abf]

Re: Help me understand

I don't know if it was a fluke or it actually made that much difference but I raised my engine up 2 holes and gained 6 mph, this was on a Sea-Nymph 16' bass style boat with an '81 Evinrude using gps.

I didn't change anything else and both runs were upstream with the same load so up and out is the thing to do and is the easiest way to gain speed.

The next easiest is redistributing weight, most people put the gas, battery, cooler and themselves all in the very back of the boat, you should try to have the load spread though out the boat so at rest it just tilts to the rear.

Watch your rpm, if you are running the original 9.9 prop and upgraded to 15 you almost have to be overevving, get a tiny tach and see where you are, you will need a tach anyways to prop it correctly, you have to know where you are to know where to go with a new prop.

And last, possibly a foil or tabs if they make them for a boat as small as yours

And I kow it hard but don't get to crazy with the speed, once you get the easy ones, motor height and load distribution adn prop it is very very hard to get speed on the water, 25mph in a small tinny like that is plenty fast anyhow.

 

[Home Cookin']

Re: Help me understand

on the pictures above of the home-made jack plate, the motor is still too low.

you have a 13' jon boat with a 9.9. Speed is not its purpose, so whether it goes 21 or 24 is irrelevant.

You also probably don't need 12 gallons of gas for a boat and motor that size. Downsize to at least a 6, or two 3's. Carry a jerry can if you must. You can then focus on reducing and redistributing weight, which is the primary means to adjust speed and trim on a small boat.

 

[jbjennings]

Re: Help me understand

22mph with a 9.9hp motor and 12 gal of fuel, a battery and trolling motor, is all you can possibly hope to get out of it. Be happy with what you have! If you want to go faster than that, you need a bigger boat with a larger outboard. Any faster in the boat that you have and you will eventually wind up with that boat pitching you in the water. Don't ask me how I know! Weight really kills speed. I'm wondering why you need 12 gallons, too! You must be really going a long way to need that much fuel, and for that you would be far better off with a bigger boat and more hp.
JMO,
JBJ

 

[steelespike]

Re: Help me understand

The 9.9 and 15 came with the same size props.
The 15 is rated at a max of 7,000 1000 higher than the 9.9.
A 9.9 will run about 3 hours on 3 gallons at wot.
I couldn't find a 1332 and the present day 32" jons are rated around 3 to 7 hp.
I wonder about yours?

 

[scanman]

Re: Help me understand

A flat bottom has lots of drag. How much hook?

Not sure what "hook" is

 

[scanman]

Re: Help me understand

I really doubt that you increased the hp by 80% with just a carb and reed change. The 15hp has a different cylinderhead than the 9.9 does, was that replaced also?

My guess is that your motor is closer to 12-14 hp with just the carb change and reeds.

A 9.9 will push a 14' boat around 20-23mph and a 15 will do just a few mph better.

I was under the impression that the 9.9 & 15 were exactly the same, save the carb between '74 & '76. That's kinda why I was thinkin I'm about 18hp. Sounds kinda unreasonable to me too.

 

[scanman]

Re: Help me understand

Sounds more like a canoe than a boat. I would think that the more air and fuel in also needs more out. Did you change the exhaust any?
What prop you running right now? the old 9.9 or a 15hp one. more hp needs to be transferred to the water with a bigger prop

LOL, sometimes it feels a lot like a canoe! I thought the exhaust tube was the same for both motors. The ones I've seen for those years appear to be. I'm still runnin the stock 9.9 prop, & I'm thinkin the 15 prop would keep me from overreving it.?.

 

[scanman]

Re: Help me understand

While I know speed isn't it's purpose, I'm really lovin goin fast in it! It doesn't really get "squirrelly" right now with the bow down attitude on the pad. The only things up front are the troller & battery, & the weight seems to be well distributed. I bought the 12gal tank while I was workin for BP lookin for oil inshore, runnin 8 hrs. a day. Never burned it all, but it was nice to know I wouldn't run out & have to be towed while workin. We can take this lil boat out as far as the barrier islands on a good slick day, & I like havin the fuel available to go from Pascagoula to Biloxi on the water IF I wanna. It also takes about 30 min. of idling to get outta the Bayou from the ramp at my house. I still have my 6gal. tank I can use for speed runs if necessary. I can't help but want all the speed it'll give me, but I'm very careful about applyin the throttle 'till I know how she's gonna act. I appreciate al you guys concern for my safety. Thanks.

 

[scanman]

Re: Help me understand

The 9.9 and 15 came with the same size props.
The 15 is rated at a max of 7,000 1000 higher than the 9.9.
A 9.9 will run about 3 hours on 3 gallons at wot.
I couldn't find a 1332 and the present day 32" jons are rated around 3 to 7 hp.
I wonder about yours?

I kinda wonder too, LOL! It says 9.9 on the transom tag. I know cause I rebuilt the transom 2yrs ago, & was careful to replace the tag. It was built in 1968 though! I wonder if the ratings were figured differently back then? The overall length, measured from top of transom to top of bow is 13'6". It was made by a company called DeSoto MFG. Not sure about thier history, & I've not seen another one like this, but IT DOESN'T LEAK!

 

[Dhadley]

Re: Help me understand

If you put a straight edge on the running surface you'll see the hull has a slight downturn or hook right at the back. Use at least a 4' straight edge on the bottom and you'll see some daylight right at the back, probably an 1/8" or so. The hook helps you get on plane but will cause drag and keep the nose down. Kind of a trade off thing.