76.1 mph.. How can I gain 4 mph?

[jeepster21]

Last winter I got bored and turned my 1971 19 ftTahiti Jet boat into an outboard powered boat. It is powered by a 250xs Mercury optimax, it has the torque master gear case with 1.75 ratio.I bought a 27 mirage plus prop and took it for a spin. It hit 76.1 on the GPS. I was hoping for 80mph..
I know I can hit 80 but I cant control the boat past 76.1 mph, it starts to chine walk and gets to the point it wants to throw me out and I cant steer out of it.
The gas tank is in front of the boat under the bow, I moved it to the back to see if I could get it to settle down but it made it worse.
Im pretty sure the round bottom is the culprit.
Is there any thing I can do to make it handle a little better with out taking the boat apart and flipping the boat over and building a pad of some sort?
These are the only pics I have right now.

[Forkin' Crazy]

Looks like a fun boat, but I'm afraid your going to have a flat pad to get any more out of it. I know a few that tried and have done the same as you and once you get it aired out you can't control it. The jet pump has a ride plate for that.

[rkmirage]

Lucky to be able to get that speed. Either flip it and put a pad on it or use it as a cool looking 70mph boat. You can't drive through chine walk and you can lose control trying. Cool looking boat!

[native2]

It looks like a 25 inch motor. That will contribute to your chinawok....

[native2]

Put a set of lenco trim tabs on the back. Dragging those around might be a cheaper/easier alternative than adding a pad. Will scrub off some speed, but at least you can drive it...............

[rkmirage]

Cawley is right about the 25 in. mid, still, the round bottom is the problem, it will help if you put a 20in on it with lwp and raise it until the prop shaft is just below the bottom. I still don't think you'll see 80 without getting wet.

[whipper]

Sand the last two feet of the keel to the stern in a Vee. Narrower to wider aft. Its OK to be a little rounded but very slight. The width at the transom make it around 8 inches wide. Then get low water pickups and learn how to drive. Meaning youll be so much faster theres going to be chine walk untill you master it so theres not anymore. Ballancing your hull by adding counter weights in the port gunnel to around 30-50 lbs should do it. With a 30 pitch prop you should be north of 90+ with that setup. Very tricky at those speeds letting off so work your way up to it. Youll maybe see 80-85 at the most before you loose water pressure without LWP,s as your prop shaft hight with a pad should be around 1/4 below to even with the pad. You will always need to wear your PDF as above 80 if somthing happens your going to need it. Thats what I would do with your setup. OR I would by a 18-20fter to hang that motor off. Theres a mint Baker Tunnel that your 250XS would hang nicley off on hear for less than 2000 with trailer if someone buys his motor first? Theres options like that. But your vee would be really really fast with a pad and LWP done right. Not hard to do. Just get some pics or go look at an Allisons pad and make something close. You can always tweek it once you have the basic pad done after water testing. LWP for sure though even if you keep her as is would be the first thing i would do. That should get ya 4mph and a 32 drag4 otta get ya there also. Just be safe, small boat big motor fun fun fun but can bite ya so just be alert all all times. It looks like your around 2-3 inches below? raise your motor to 1inch below at nutral trim and see if you still get good water pressure? if you do that will increase speed 3-4 mph also. Thats why I say LWP then you can go even higher and be faster as is. But a pad with the pick ups would yield the mostest.

[Storz]

Trim tabs?

[croSSed]

What kind of steering do you have? I switched from single cable to hydraulic and the ability to drive through the chine walk (and, BTW, you CAN drive through chine walk w/ good steering system, solid mounts, and SEAT TIME) increased dramatically. My boat still wants to walk side to side, but it is totally controllable w/ hydraulic steering and solid mounts. My boat sees 72.5 mph @ 6400 rpm w/ a 26P Laser II, NO lwpu, and only a 2L XR2 150. My boat is not TOO dissimilar to yours, w/ the excepting being that my Avenger has a "sort-of" pad. I'll post a pic later if you want to see what I mean.

[rkmirage]

You CAN'T drive through chine walk. You have to prevent it with steering input. Chine walk will only increase with speed, it not a transition like hop.

[Mr. Demeanor]

The easiest way to get to 80mph is to take your handheld gps in your hand and at top speed, swing your hand forward as hard as you can like your going to throw it but dont. Its usually good for about 3-4mph.

[jeepster21]

Yes it is a 25 inch motor, but im not gonna complain since I only payed $750 for it, but I am looking for a 20 inch mid.The gear case has the lwp on it but no nose cone. The boat has dual telaflex cables with solid motor mounts. Ill have to look at the bottom of my brothers xb2003 and see if I can make mine look like that. The boat is very stable till about 72 then it adds a little pucker factor after that. If I put about 4 people in the boat I can keep it floored and still do about 73 ish and that seems to be fast enough to scare them. I have raised the motor since I took the pictures the prop shaft is about an inch below bottom now. I am just thinking of leaving it alone but it all depends on the free time and money I have this winter.

[croSSed]

You CAN'T drive through chine walk. You have to prevent it with steering input. Chine walk will only increase with speed, it not a transition like hop.

I guess it comes down to what one's definition of "drive through" chine walk is. I guess my definition has been if your boat will chine walk without you doing anything to correct it, then when you DO drive so as to correct the chine walk, that is "driving through it". My boat will chine walk at full speed if I don't take steering measures to counteract it. To me - but I guess I'm wrong? - that is driving through chine walk.

[whipper]

Drive threw is a expesstion no big deal. Some Ive heard say this in a way that might be confused with if you start chin walking just floor it and it will go away on its own. As we all know thats not the case. So for saftey resons most just saw with seat time youll learn to drive without any chine walk. Muuch the saame as you describe sayine your will if you dont take corrective action to avoid this. Your not wrong in your description I think RK was just thinking saftey so as not make anyone reading this thread think with there 300 hp 18ft vee an just trim her out and floor it and all will be well.

Hey Jeepster sounds like your setup pretty good. What does your hull weigh any idea? No big deal going up a notch on the plate unless your already on the top holes? 1 inch is good though. If your ever decide to try a pad youll be able to go higher as your taking more off the keel depth. The more ya take it down the high you can go in threory. The flatter keel as a pad should alow the bow to run a little higher and thuss the hole hull. The pad should be just wide enough to suport the transone from wanting to {sink} as you dig a hole with your prop if that makes sence?LOL The wider area will help suport the stern at speeds determand by the width. You ill loose a very small rough water smoothness but your not going to really notice that because if its rough youll be slower with the rest of the vee in the water anyway then. But for smoot and rippeled water yourll pick up a lot I think. In your sig your running nice and level. Youll be a ble to reduce a little more wetted surface as well as aid with the trust buy reducing friction with a pad. Run your hand threw water like a knife. Then run your hand over the water palm down. Thats what yor tring to accomplish. Skimming rather than burried in the water.


Since you have a power full touquy motor theres some props that lift more than others. The only 3 blades I know that lift really well for me are the OS cut by steve smith and a stock chopper11. Being semi surface type they both do like higher tramsom hights. 1 inch might work though. Another really good lifter in a 4 blade is a hydromotive T4-X and a Performance props Drag4. I would guess with your motor and setup the Drag4 and the T4-X in a 30 would be a really good prop choise that should get you on the limiter at 1 inch bellow .

[rkmirage]

I guess it comes down to what one's definition of "drive through" chine walk is. I guess my definition has been if your boat will chine walk without you doing anything to correct it, then when you DO drive so as to correct the chine walk, that is "driving through it". My boat will chine walk at full speed if I don't take steering measures to counteract it. To me - but I guess I'm wrong? - that is driving through chine walk.

Ok, as long as he doesn't get the impression that its a phase that happens on the way to top end like porpoising before laying down or airing out. I used to have an Avenger, loved it, very hard to keep up on that tiny pad! But it can be done. With a round bottom you have no chance because of nothing to balance on. Tight dual cable or hydraulic will help and solid mounts are a must, but in the end it still has nothing to balance on. 750 for that motor? That was a deal!