More Bow lift to help with broaching -Do you lift the motor up a few notches or down?

[blacklab]

I haven't had a boat that couldn't be driven right, but, my mate had one that was "bad" it was a common brand, and it didn't matter what you did, it would just sheer off to either side at near right angles, only happened if the sea was directly behind you, side on, head on quartering it was fine, but, directly behind, it was exciting to say the least, we fish a FAD that is NE from our ramp, coming back in a summer NE wind was a lucky dip.

Exactly the same as the boat I was mentioning, he spent an incredible amount of time and effort as I said with props, weight distribution etc.
Basically, whether it was the actual boat design,or trim as you mentioned, I don't honestly know, I guess there's just some plain mongrels out there !
His tactics, were basically to treat the following sea like a bar crossing, never letting the following wave catch up, he consistently had an on and off the throttle type approach, and I mean fair putting the throttle down.
It's an interesting scenario by all accounts, why some boats perform like this,, that by all accounts perform well apart from that situation and as mentioned, it used to fair frighten the sh""te out of you at the same time I don't mind admitting.

[Danstu]

Thanks for all the comments appreciate everyone's thoughts. To answer some of the questions yes I have Volvo penta trim tabs, yes I put all the weight at the back I can which includes a 200 litre esky half full of water and ice and occasionally some fish!, fill the live bait tank up and I also have a 25kg block of lead that I put in the back starboard corner. If I have two people sitting on the port side I put all this weight in the back starboard corner and still have to drop the starboard corner trim tab down to level the boat. On our last run back to Mooloolaba I did run a long way north and then pulled back south so that it was a straight following sea rather than a 3/4 on following sea just for safety. I was also working the throttle to alleviate the odd broach. Just trying to work out if I could stop it happening altogether. My last boat was a Seafarer Victory was pretty much idiot proof in a following sea and was actually a heap of fun to drive in one. My new boat has a hard top and quite a lot of weight in glass as well so maybe too much weight forward. I know one of the plate manufacturers welds a box in under the floor at the transom which he fills with lead to try to make them more bum heavy. Maybe the full closed in hardtops on these boats make them to nose heavy.
Thanks for the comments everyone.

[Dignity]

So a 7.2m plate ??? Brand might help, plenty of plate owners up here, it would help to know the vessel.

[Moonlighter]

Some hulls are just poorly designed -not saying that yours is, of course. If you do some research there are particular hull characteristics to do with beam amidships vs stern beam that almost guarantee broaching in a following sea.

I had an old Quintrex Seaman MkI as my first boat, and it was an absolute shocker in a following sea, and even when you crossed a wake off a cruiser heading in the same direction, it was likely to turn hard right with very little provocation. Had to be bloody careful!

One of the issues with plateys is that some of them were designed and built by welders who know nothing about hull design - remember those plate boats used up around Thursday Island that sank with several lives lost? Case in point.

Many of the better plate builders have now had their designs updated or designed from the ground up by naval archtects and the result will be that they behave predicatably in all seas.

Having said that, you mentioned the issue of engine height. An engine set too deep can turn a great hull into a dog. Ive not heard about it causing broaching, but it is a cause of porpoising and other nasty habits and generally can make a hull plow too.

So regardless, it is well worth taking the time to get the engine height set up right before playing with props or doing other things like adding foils. On a smooth bit of water, get up to a good cruise speed, trim out to the optimum position where she runs sweet, then get someone reliable to hold the wheel while you go down the stern and look over at the engine leg to see where the anti-vent plate is vis a vis the water surface. It should be skimming the surface, not buried deep in the flow. If its buried, then lift it till its skimming, getting splashed is fine, but not buried under in the solid flow.

[catshark]

install full length heavy duty skeg keel. getting pushed around in 4 foot chop. maybe your cabin is a handicap in weight and position.

[Andy56]

Some hulls are just poorly designed -not saying that yours is, of course. If you do some research there are particular hull characteristics to do with beam amidships vs stern beam that almost guarantee broaching in a following sea.

I had an old Quintrex Seaman MkI as my first boat, and it was an absolute shocker in a following sea, and even when you crossed a wake off a cruiser heading in the same direction, it was likely to turn hard right with very little provocation. Had to be bloody careful!

One of the issues with plateys is that some of them were designed and built by welders who know nothing about hull design - remember those plate boats used up around Thursday Island that sank with several lives lost? Case in point.

Many of the better plate builders have now had their designs updated or designed from the ground up by naval archtects and the result will be that they behave predicatably in all seas.

Having said that, you mentioned the issue of engine height. An engine set too deep can turn a great hull into a dog. Ive not heard about it causing broaching, but it is a cause of porpoising and other nasty habits and generally can make a hull plow too.

So regardless, it is well worth taking the time to get the engine height set up right before playing with props or doing other things like adding foils. On a smooth bit of water, get up to a good cruise speed, trim out to the optimum position where she runs sweet, then get someone reliable to hold the wheel while you go down the stern and look over at the engine leg to see where the anti-vent plate is vis a vis the water surface. It should be skimming the surface, not buried deep in the flow. If its buried, then lift it till its skimming, getting splashed is fine, but not buried under in the solid flow.

that would be my approach too. Get motor height right first and foremost. At least then you have the max control over the hulls attitude. Secondly, i would go for a 4 blade prop just for the control it gives you. I assume you dont do water skiing, lol. A standard 3blade prop is more an outright speed lover. A 4 blade will tend to hang on more and provide more torque in the lower midrange.
Get the center of gravity/ pivot point working for you, not fighting you. i would look at weight distribution with the understanding that you are trying to get the pivot point in balance. Too much either direction and you lose control of the balance. This could be a design fault too with a pivot point badly placed on the hull/motor combo.
A jackplate is one way of altering the center of gravity back towards the stern. They come in many different offsets such as 4", 6" and 10". If you feel the hardtop is too much weight up front, this may just work. But then, you would go through the setup with the jackplate on first. The more expensive and rather dubious way is to use a pod. Unlike a jackplate, a pod is fixed and adds bouyancy where it may not be needed ie moving the center of gravity forwards.

to me, this gives you max possible control of the boats attitude by using the motor trim. You should be able to keep the bow up or down on demand. Shifting the weight around to keep the bow up or down is fixing the symptoms rather than attacking the cause.

other issues, you have no control over. Things like underpowered boat or bad design.

if you experiment with no understanding, you could easily end up with a motor too low and too much weight on the rear . mistaking symptoms for causes and giving you no real control.

[Andy56]

that would be my approach too. Get motor height right first and foremost. At least then you have the max control over the hulls attitude. Secondly, i would go for a 4 blade prop just for the control it gives you. I assume you dont do water skiing, lol. A standard 3blade prop is more an outright speed lover. A 4 blade will tend to hang on more and provide more torque in the lower midrange.
Get the center of gravity/ pivot point working for you, not fighting you. i would look at weight distribution with the understanding that you are trying to get the pivot point in balance. Too much either direction and you lose control of the balance. This could be a design fault too with a pivot point badly placed on the hull/motor combo.
A jackplate is one way of altering the center of gravity back towards the stern. They come in many different offsets such as 4", 6" and 10". If you feel the hardtop is too much weight up front, this may just work. But then, you would go through the setup with the jackplate on first. The more expensive and rather dubious way is to use a pod. Unlike a jackplate, a pod is fixed and adds bouyancy where it may not be needed ie moving the center of gravity forwards.

to me, this gives you max possible control of the boats attitude by using the motor trim. You should be able to keep the bow up or down on demand. Shifting the weight around to keep the bow up or down is fixing the symptoms rather than attacking the cause.

other issues, you have no control over. Things like underpowered boat or bad design.

if you experiment with no understanding, you could easily end up with a motor too low and too much weight on the rear . mistaking symptoms for causes and giving you no real control.

Been dreaming about this issue all night , lol.

if the pivot point /fulcrom is too far forward, your motor is working overtime to maintain " control" bcause its acting against forces its not meant to . Any force from behind maybe strong enough to over power what ever control the motor has. So the " common sense" approach is to add more rear weight to get the bow up. This only plants the boat further in the water and making it even harder for the motor.

Back to the sea saw analogy. If one side of the sea saw is longer than the other, what do you think happens? If the motor is on the short end, it takes a hell of a lot more effort to be able to control the balance. With a large enough motor, you may not even notice. Like wise, if the motor is on the long side, the motor has too much "sloppy" control. Rear wheel steering. ( another analogy) Any sort of following sea can easily overtake your control. Just the rocking motion can do it.
Being out of balance also means minimal real trim control. You can trim in and out with some effect but no real control of the bow.
That would be my explanation for whats happening.

[GBC]

4 bladed props give more stern lift and prevent bow lift which is unpreferred in a broachy boat. What brand and model is the boat? Some info would be good.

[Andy56]

The prop itself doesnt give you lift. It can give you lift if you dial it in. A 4 blade has more purchase on the water, bow control is what you do with the extra grip. If boat is out of balance to start with, of coarse you wont get the desired response.

[GBC]

Righto. You know best.

For the OP so he doesn't get confused or make a poor decision based on poor advice:

https://www.sportfishingmag.com/prop...-blade-props-0

I just remembered why I don't bother too much on this forum anymore.

[Andy56]

Righto. You know best.

For the OP so he doesn't get confused or make a poor decision based on poor advice:

https://www.sportfishingmag.com/prop...-blade-props-0

I just remembered why I don't bother too much on this forum anymore.

I think you should read the whole thread in context. That article makes assumptions. If you want , i can make a list of them . Of coarse if you only put a 4 blade prop on it will do as you say. But please, dont put words in my mouth. We are talking about an overall setup situation here, not just a prop.
I think sometimes things get complicated so you need to go back to fundamentals and try to understand whats actually going on rather than quote articles as above. It just seems you jumped on me without taking everything in context.

My whole thrust is to try to understand whats going on, not simple remedies that havent worked or mantras that everyone subscribes to without even thinking. Knowing what the real problem is allows you to find a solution.

[Andy56]

If you think i am just opening my mouth, read this article

https://microship.com/balance-your-b...vity-database/

you dont have to read the whole article to understand how important it is to get the setup right.

the relevant part is this The Principle of Moments… A Few CG Basics

Every boat has a center of buoyancy (CB), which is the center of the underwater volume of the vessel. She also has a center of gravity (CG) which is where all the mass would be concentrated if it had to be compressed to a single point. If the boat is to float properly on her design waterline, then the CG must be in line vertically with the CB…. if it’s not, then the boat will correct for it by changing trim (and thus underwater shape) until the new CB is in a vertical line with the CG.
This pretty well sums up the problem. There’s not much you can do about the CB for a given boat (which wanders around with heel angle, but we won’t worry about that since it’s out of our control), and the CG of the raw hull is pretty much a given as well. But the moment you start installing equipment and people it all changes… often dramatically. Just take a stroll from bow to stern of a canoe for a quick demonstration of how much your weight can affect the trim of something with 1,000 pounds or more of buoyancy.

[Andy56]

Here is another analogy.
You have a ute and put a pallet on the tailgate and the nose goes up. What would you do to compensate? A/ put lots of weight on the bullbar B/ move the pallet to the middle of the ute?
A / ####s up the handling big time. Sure it rides flat but your xtreems are in control. Why do you think sports car manufactures try to have most of the weight between the axles ?
So when the advice is shift more weight up front, understand what the dynamics at play are. If the boat is out of balance to start with, your just complicating things.

[gofishin]

I like 4 blade props very much in offshore boats, and have tried many different types on boats, and many different boats, as well as the same with 3 blade props.

4 blade props nearly always provide more transom lift - and in your case would be one of the last things I would try, and only in combination with/or after implementing other corrections that have worked/solved the problem.

Unfortunately this problem is not that uncommon in the big platey world with the lesser known (developed/tested) brands. Is it a new boat? If so why isn’t the builder involved? Weight distribution (design) problem and correction is best solution is to correct these problems.

Lead ballast etc is a bandaid but is sometimes unavoidable as consumable ballast (fuel/water/food/ice) is not always there. Eskies full of water or people etc down the back is not the answer either. Our deckies come out to enjoy boating and fishing too .

Where are all your batteries? 1 or 2 motors? Boat stats weight etc??? Hard for people to help when they don’t know all the facts.

It is always best for performance/economy to have the AV/cav plate skimming or even above the water even (as you can with many 4 bld props ) but unfortunately in your case an optimum positioned plate is NOT what you want.

To create bow lift, as one or ‘part of’ the solution for a broaching boat, you need to LOWER your motor/ bury the plate, and this is used quite often to mask a design problem resulting in broaching. This has many downsides with speed, economy, spray, porpoising in some cases etc but does create bow lift without any ‘out trim’ in comparison to a motor at the correct height. Why? Look at the physics.

However check the plate level first, it could already be buried somewhat to mask the problem!

Find a proper solution with design and installed weight distribution, and if this doesn’t entirely solve the problem leave motor(s) ‘a little’ buried.

Cheers
Brendon




Sent from my iPad using Ausfish mobile app

[robothefisho]

I would try high rake 3 blades to start with. What motor is it? Another issue to rectify the design would be weld on a lifting strake.