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nzjetboating.com Forums => Tech Library => Hulls => Topic started by: chippy7 on August 11, 2015, 18:54:06 PM

Title: planing strakes on large jet
Post by: chippy7 on August 11, 2015, 18:54:06 PM
Hi , i am wondering if any one out there with experience with bigger hulls can tell me if planing strakes would help much with bow steer on our sea boat. we have a 9 mtr jet with twin 300hp diesels running through 2 Hamilton 213 units , the boat is about 3.5 ton depending on fuel levels (700 l capacity) . it has always been a bit of a dog for steering but have ripped out hydraulic steering and changed tanks to shift weight forward . it has two pieces of alloy approx 100 by 10 and about 600 long welded outside the units like surfboard fins sticking down at right angles to the hull and we have added another 300 odd mm as a full trim tab across the transom too . and the nozzles have been wedged .its a fine line as the way it was you couldn't steer straight up a flat lake and now it bow steers bad off even  a ski wake. thinking about adding strakes to help with a bit of grip up the front but keen to hear any opinions on it ,
 cheers in advance
Title: Re: planing strakes on large jet
Post by: mohawk on August 11, 2015, 19:19:41 PM
Shifting weight forward will only make the bow steering worse . Raise the nose too much and it will porpise and lower it too much and it will bow steer like a pig.
Strakes will lift the hull , but I don't think they will help you much , its a trim issue , not a strake issue.
Title: Re: planing strakes on large jet
Post by: Matthewf/Norbert/TuiDog on August 11, 2015, 19:43:20 PM
what he said.

hydraulic forces are very hard to fight with nozzles and strakes, many have tried.

get weight backwards.... test with 10 mates standing at the transom.. but if you have lots of weight moved fwd, it will be tough to fight.

hull may also not be flat underneath.. check for a concave
Title: Re: planing strakes on large jet
Post by: mohawk on August 11, 2015, 20:20:40 PM
yup , forgot too mention LOOK FOR HOOK  b>
Try 2 large 40gal plastic drums full of water at the back and stuff all gas , if your like me you probly don't have 10 mates  >>@
Title: Re: planing strakes on large jet
Post by: Nigel on August 11, 2015, 20:24:47 PM
Got some pics of the hull, transom etc please
Title: Re: planing strakes on large jet
Post by: Paul on August 11, 2015, 20:32:20 PM
Chippy, what speed is this happening at?  A twin 300HP pushing a 3.5T is probably not going to be massively fast - is it still displacement or a clean plane?  What is minimum clean plane speed and what's max speed?  Does the issue get better (i.e. the boat has better steering control) with higher speed?

Does it drift more one way than the other?  Could it be, that as it's a twin engine, that one engine or unit has worn more than the other, and the thrust is no longer balanced, so you are needing steering input to try and bring it back to the middle?  Maybe try just using different throttles levels (port vs starboard) and see if slightly uneven throttle positions make it worse or better.
Title: Re: planing strakes on large jet
Post by: chippy7 on August 11, 2015, 22:02:31 PM
hi ,cheers for the replies . the two tanks up the front are reserve tanks of about 70l each and only filled when heading offshore. there is a tonne in the motors and units and the way it was you had the bow right up in the air.
Paul - its a clean plane , the boat tops out at 3800 rpm and gives you 60kph. most of the time we are at 3000 rpm and thats about 40kph according to the gps .  much lower rpm than that and she wallows back of the plane quite quickly .
we had the boat  cutsom  built new about 10 years ago with all new running gear and only has about 220 hrs on it now .  running the motors at different rpm makes no real difference until it slips off the plane with only one motor going full rpm.
 its been ok but you have to constantly be tweeking the wheel to keep her straight . and it can be a bit of fun in a big following sea when she knifes off with bow steer !!will get some photos up and look for some mates!!
Title: Re: planing strakes on large jet
Post by: Jimmy Jet on August 11, 2015, 22:12:45 PM
Hi Chippy,
I worked on a large private yacht and one of the tenders was of very similar size and weight but with slightly more power and slightly bigger jets (292s from memory). It too didnt like holding a straight line very well which was a hassle on long passages.
I agree with the other guys in that you have too much weight forward. Bow steer is going to give you much more of a problem than not being able to hold a straight line.
We installed fins as you already have but about 200mm long and it helped heaps. Shallow water operation wasn't a requirement but safety around swimmers and divers was. The trim tab will be lifting the stern as well.
Id be getting the weight back aft maybe using temporary tanks to find best balance.
Title: Re: planing strakes on large jet
Post by: chippy7 on August 11, 2015, 22:41:25 PM
Hi , here are the only photos i have of it , will take some more next time I'm near it
 cheers
Title: Re: planing strakes on large jet
Post by: PM on August 12, 2015, 09:22:54 AM
What's the deadrise at the transom?

Warped or constant deadrise?

Where was the boat built?

My first inclination is to add strakes to the front 1/3rd of the hull, and one pair running right to the transom outside the jet intakes. 3.5t is a lot of boat, and moving weight forward may well upset the planning balance. If the nose spends to much time in the air with the weight back I'd be thinking about tabs rather then weight redistribution.

I've never been a big fan of using skegs under the transom to try and maintain a straight line, it is an idea borrowed from the ski-boat market to increase lateral drag as they have relatively flat bottoms to reduce wake. Even so, they are normally added to the boats in pairs, one at the transom and another usually just aft of the centre of rotation. Possibly an option for you but it does run the risk of boiling the water leading into the intake of one of your jets in a turn.

Paul, I doub't it'd be unbalanced jets as the thrust difference would have to be reasonably large to create any meaningful torque, particularly given the distance the thrust is from the centre line of the vessel.
Title: Re: planing strakes on large jet
Post by: chippy7 on August 22, 2015, 18:35:57 PM
hi , finally got out to get some photos of the boat today.
 PM the boat looks to be variable deadrise , starting at about 18 to 20 deg at transom and up to 25 odd . bit hard to tell from photo as boat is backed up to back of shed . it was built in lower south island but prefer not to say as it didn't end that well with builder. trim tabs have been added to extra piece that's been put on across back of hull. will take out nozzle wedges for a try next time out
 cheers and thanks for all the replies
Title: Re: planing strakes on large jet
Post by: chippy7 on August 22, 2015, 18:45:11 PM
here are some
Title: Re: planing strakes on large jet
Post by: chippy7 on August 22, 2015, 18:46:54 PM
and more
Title: Re: planing strakes on large jet
Post by: bbs 540 on November 16, 2015, 21:42:43 PM
 A couple of strakes in the bow will make it less knifey and will hold the nose up but will make it pound in rougher conditions. Don't run your stakes all to the back if you just want to lift the nose. Good advice about testing moving you weight back try the easy stuff first.