violent porpising ( cavitation )...... ok maybe not violent

[OutANoWhere]

i have a 1997 mako 181 flats boat (18 ft.) with merc. 135,4 blade prop,hyd. jack plate,bennent trim tabs.porpising is very uncomforable and annoying,it seems i am constanly adjusting the tabs,the trim,the engine height (via jack plate),and etc. to try and correct/compensate this annoying feature that came with my recent purchase.it seems to be tolerable at wot and as soon as a slight and i mean very slight decrese in throttle is made she begins signifigant repeditive rise and fall in the bow i have searched the web for answers/soulutions but to no avail. please help. one thing that may be of signifigence is....i had a trolling motor installed and my mechanic thinks that i may have a prop problem,i have a 4 blade 13 3/4 x18 stainless and while he was examining it to determine the pitch he noticed hairline cracks in 2 different blades i know this needs to be fixed or replaced but dont know if this could cause any of my problems. thanks

 

[DJ]

Re: violent porpising ( cavitation )...... ok maybe not violent

The REAL answer is too much weight in the stern, or a hook in the hull.<br /><br />Just the engine, jack plate, etc, on the stern is plenty of weight.<br /><br />You have to look at moving whatever you can forward: batteries, etc.

 

[walleyehed]

Re: violent porpising ( cavitation )...... ok maybe not violent

Trolling motor on the bow or kicker on the rear?<br />#2., what manufacture and model of prop?<br />#3., How do you trim out-what is the process you use?

 

[Dhadley]

Re: violent porpising ( cavitation )...... ok maybe not violent

Porpising with that hull is very common. Yours isnt doing anything many others dont. Yes, there are band-aid fixes but DJ has hit on the solution -- youll have to work on the hull to get rid of the problem without sacrificing efficiency. <br /><br />Those hulls tend to have a lot of hook to get them on plane quickly with small motors. Every time your prop and set up try to lift the hull, the hook tries to set it back down.

 

[swist]

Re: violent porpising ( cavitation )...... ok maybe not violent

 

[swist]

Re: violent porpising ( cavitation )...... ok maybe not violent

My boat porpoises at the slightes provocation. If I trim it in a little it will usually stop, but I can tell by the rpm's that it's not where it wants to be otherwise.<br /><br />I have come to the same concusion as the other replies. I have too big an engine (225) on a bracket. Brackets are nice for some things but this is a big downside. Fiddling with things over the 11 years I've had the boat has only convinced me even more that too much weight is too far back.<br /><br />And now I'm soon faced with a repower situation and every new EPA-2006 engine of that HP (be it 4-stroke or direct-injection) weighs at least a little more than my current rig, and a LOT more in some cases. So I'm hosed. <br /><br />The moral of the story is to think twice about the weight distribution on your boat.

 

[Kiekhaeferscurse]

Re: violent porpising ( cavitation )...... ok maybe not violent

Outanowhere & swist cant you remove the setback brackets to try and get the engine wieght forward a bit?

 

[OutANoWhere]

Re: violent porpising ( cavitation )...... ok maybe not violent

thanks to everyone who responded. let me address all of you (5).DJ,i dont think i have excess weight in the rear 3 batterys in the console,35gallons of fuel in the bow(right where you step up on the deck from the cockpit)my baitwell is in the rear but it does it emty or full,no hook in the hull,if anything it a slight drop when i straight edge the bottom its flat all the way back until the last 3 inches it is slightly away from the straightedge.Walleyehed i have a bow mounted trolling motor,im not sure of the prop manuf. it has a yellow decal right on the blades but i cant make out what it says at this point,and it doesnt matter how i trim out once she gets on plane you better be ready for a ride,and then it will settle out as long as i dont back off.(i do put the tabs down most of the time and as it gets on top i start bringing them back up however i have to have some tab just to make it tolerable)Dhadley,i dont know what you mean "work on the hull" unless you mean fix the hook which i dont have.Swist,i cant trim my motor at all it only gets worse unless i am "wot",as far as weight in the rear i really dont have anything out of the ordinary other than a jackplate which i dont think was a factory install but its not like its a paralift that would set the motor back farther than a bobs j/p which is what i have and the motor is a 135 and the boat is rated for a 150.one more thing on "hook"i am under the impression that hook is when the hull curves down slightly to assist in plane like a trim tab so if that is what a hook is i dont have one please read this and give me more feed back if you fellas have some suggestions. i appreciate your previous replies.thanks

 

[Jdeagro]

Re: violent porpising ( cavitation )...... ok maybe not violent

I think what DHadley is trying to say is to create better balance in the hull. We have a number of customers including the Bass Pro Shop Boat centers who experience the same problems with this boat. They typically install Trim Tabs, some helm controlled and some Smart Tabs. The trim tabs add more lift to the stern and keep the bow down, but when combined with the hook in the hull they can actually exaggerate the problem if they are deployed too much (helm controlled). The same goes for our Smart Tabs if they are adjusted with too much pressure, but in this instance they can be adjusted with less pressure easily. What happens with the excessive lift is that the bow is held bow when on plane and with more and more force as the boat increases in speed (due to the hook in the hull). If the bow is pushed down too hard (especially with too much tab deployment) the water can squeeze the bow back up as the water conditions (waves for example) change. Then the hook and tabs push the bow back down and the cycle repeats. You are correct, the tabs will need to be adjusted with nearly every change in direction, speed, and water condition. It is a hull design issue!<br /><br />Without some major work on the hull itself, the best thing you can do is to try balancing the boat better with changes in weight distribution.<br /><br />In my opinion the recommendation for changing the prop is not at all valid. There is too much emphasis on props to solve handling problems, that is not the purpose of the prop.

 

[swist]

Re: violent porpising ( cavitation )...... ok maybe not violent

Outanowhere & swist cant you remove the setback brackets to try and get the engine wieght forward a bit?

This is a permanent Gil bracket. You can't remove it or there's no place to hang the engine. Nor is the position at which the engine is mounted adjustable - it's all the way back. And even if a shorter bracket were available at reasonable cost (NOT!), the engine could no longer be tilted up fully as the cowling would hit the transom.