Marinette cruiser hull extension

Mods and custom builds
Msmithabr
Posts: 3
Joined: Thu Aug 18, 2022 6:46 pm
1

Marinette cruiser hull extension

#1

Post by Msmithabr »

I've just started a new project. A 1960 Marinette family cruiser. 28ftx9. Wanting to go inboard diesel and surface drive. Wondering if anybody has any experience in stretching these hulls a few feet. Or any similar sized hull for that matter. Any handling/trim issues? I would like a bit more rear deck space without giving up the inline 6. 

Thanks,
Mike
 
 
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kmorin
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Re: Marinette cruiser hull extension

#2

Post by kmorin »

msmithabr,

I've extended quite a few hulls of this rough size, but all were welded hull's (or plastic hull add-ons) and this one appears to be (at least) partially riveted?  Don't know that for sure as the pics aren't zoomed in on the surfaces of the hull all that closely?

In this building material, my experience in working on welded hulls' extensions won't be all that useful to your project. However, if the buttock lines of the hull bottom can be maintained as fair extensions of the original the hull will plane w/o problem. You could choose to extend the hull by removing the original transom and adding on.... or by cutting the deck and topsides out @ the original and leaving a large cross section transverse frame made up of the original transom, and extending beyond that with new material?

I have also been involved directly and indirectly extending both plastic and metal hulls where the hull extension didn't manage to keep the buttock lines fair- they angled up or down and in both cases the original hull had a change in performance that needed to corrected due to the failure to keep these key hull lines/intersections fair to their original shapes- determined by the original and forward sections of the hull.

good luck w your project.

Cheers,
Kevin Morin
Kenai, AK

 
kmorin
Msmithabr
Posts: 3
Joined: Thu Aug 18, 2022 6:46 pm
1

Re: Marinette cruiser hull extension

#3

Post by Msmithabr »

The hull is actually welded. Welded in 1960 mind you. So I'll be going through and adding some strength in key places that wasn't needed at the boats intended cruise speed. Leaving the original transom as a structural member had crossed my mind, but it's just single wall 1/8 inch aluminum. Appears they used a rear seating area to add rigidity as the transom never saw load in the original inboard drive layout. 

This boat is getting a 5.9 diesel (probably not the one most would expect) and an arenson drive. So the transom area needs some work to put it mildly. I'm also removing the original external keel skeg in favor of a keel structure inside the hull.  Deadrise at the transom is at 5 degrees and I want her to go shallow. That's the reason for the drive arrangement. I'm told I need cg close to 25% of lwl from the transom and a bit of rocker to make the drive function properly. Another reason for the engine moving aft. 

Most of the flare happens at the gunnels. I believe the wetted surface is only gaining 1/4" per foot in width on the way to the bow. So it should be relatively easy to maintain shape and fairness. My greatest fear was lack of stability. 90 inches of wetted surface at the transom. Would tapper to 89ish with a three foot extension.  I have no boat design experience and would like to feel safe with family on board 

I'm very grateful you chimed in with your experience. Any other pointers would be much appreciated as well. I was beginning to think nobody was using forums anymore. 

Thanks again,
Mike
 
kmorin
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Re: Marinette cruiser hull extension

#4

Post by kmorin »

Msmithabr,

I'm not lecturing, just noting that we'll need to have common terminology in order to share information about your hull and the proposed conversion to articulated surface drive from the keel boat she was designed to be.

The hull lines (of any hull) are very specific and need to be understood by the new builder/remodeler or the resulting confusion in terms & language will end up being a source of error in planning and (re)building.

I have an article on the Glen-L Forum about the lines of a hull and in advance of your planning and work; suggest you consider reading that post (article) and learning what the lines of hard chine hulls are- because our posts illustrate you're using terms somewhat at odds to conventional usage and confirm you're not going to be able to plan your work without a little more study of the basics of hulls' lines and how those shapes define the hull?

For example;  your hull has no flair; is has some minor flam, only forward of the master station as near as the photos show.  Flair is a curvature in two axis- and your hull has a conic (flam) or single DEVELOPABLE surface- not compound surface curvature.  Your hull has FLAM developed from a flat sheet surface that was cut as  part of large conic section and  pulled into a bow- there is no reverse curvature at the sheer (your term gunwale).

Image
The sketch (above) to the left shows a hull with FLAIR, or curvature in two directions/vectors.  The topsides of a hull w flair 'curls' outward as the bow topsides curve inward toward the bowstem.  The other sketch shows most 5Plate hull's bow shape- as your hull shows in the photos- the shapes are curved in a large conic section and the hull intersection line at any give Body Plan Section/Station is either very slightly curved outward (exaggerated in this sketch) or nearly a straight line in Section. 

Purpose here is to illustrate that while you may not have any boat design experience; you'll need to become familiar with some basic terms and ideas to succeed in your project's goal of stretching the hull and moving to inboard surface drive propulsion.

I used the term Buttock Lines and you used the term 'rocker' which is a curve in the buttock lines.  This shows that we're not actually going to be able to communicate- I can't help your project avoid the countless mistakes I've already made in hundreds of boats- if we can't speak the same language.

I hope you're not taking offense from this post?  I'm encouraging you to get enough (more) clarity in your understanding of how hull shapes are defined (lines of the hull) to be able to discuss your hull and the project your propose.

I'm willing to post responses to (attempt) to help you plan and execute the hull extension but encourage you to become more familiar with the terms used by hull designers and builders.

cheers,
Kevin Morin
Kenai, AK


 
 
kmorin
Msmithabr
Posts: 3
Joined: Thu Aug 18, 2022 6:46 pm
1

Re: Marinette cruiser hull extension

#5

Post by Msmithabr »

My experience, primarily, has been in process and power piping for various industries and automotive fabrication. I definitely have skipped some necessary boat specific education steps that could be considered necessary to start this project. Including terminology. I'll work on that. Having a design language is crucial to communicating. 

No offense taken at all. The "rocker" I was referring to was a term I was given by Rik wimp when he described what a hull needed to be able to run properly without the ability to run with positive trim. The hull has to have a natural bow high inclination to get the best efficiency. 

I'm sure your already familiar with all of this, I was just explaining where I had picked up the term and the context it was used in. 

I'm not sure if each sector of the marine industry shares the same terminology. The performance boating community terminology seems a bit different than the ship building terms I'm seeing online. Maybe the performance guys are more shorthand and the shipbuilding industry is more traditional? I'll do some research. Learn the lingo and return. 



Thanks,
Mike
kmorin
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Re: Marinette cruiser hull extension

#6

Post by kmorin »

Mike,

I'm glad I wasn't being seen as rude and that as a designer you already know that centerline of pipe is as key to pipe layout as the keel plane of a boat!  Everything depends on an accurately drawn or designed centerline to run pipe (I used to do pipe design for oil and gas offshore here in the Cook Inlet, in SC Alaska; ANSI 150, 300 & 600) and the keel plane is only displayed as a shape in the Profile View! (otherwise it's just a vertical [body plan] or longitudinal line [plan view]).

Dave Gerr, a marine architect on the East Coast (http://gerrmarine.com/) has some great books out and they're worth their costs to your project.  I've even purchased multiple copies at a huge discount by using the online book seller ( https://www.alibris.com/ ) so I could give copies of his books to friends and students affordably.  The Nature of Boats is a great read, and worth the list price on Amazon! 

Another marine design and wood builder Sam Devlin also has a few books out that explore the hard chine lines of sheet material boats. In his books there's a fine discussion of lines and modeling that are helpful to visualize the design considerations involved in your project. 

While I've only installed two Arneson style drives in one boat in all that I've built... I do understand they require some provisions not seen in other drives' hull shape requirements.  However, I'm not conversant in the required design elements as they'd be realized in correctly drawn hull intersection lines, and I'd say that discussion would be best held by contacting anyone who builds the drives for sale, anyone who's done some hull mod's to install these drives and owners or operators who drive boats- driven by these drive systems?

My (initial) primary concern is that you understand (explore and evaluate)  the shaping implications of rounded/curved/swept Buttock Lines (actual "rocker") as compared to an 'angle point'/hogged/raked/kinked Buttock Line change; in adding  your 3' hull extension.  Any hull extension to a planing boat could be a Profile View - flat, straight and level extension to the exiting hull (ignore plan view ['width'] for this consideration) or it could have Profile View (elevation view) that is rounded as the Buttock Lines rise in curves aft. 

In general, while the Arneson's may benefit from some degree of rocker in the Buttock Lines aft??  That point must be explored very exactly to agree with the person who's asserted this claim.

Another point to make in regard pitching the bow upward, semi-permanently, is that the running waterline will move aft to amidships and this hull's V/deadrise/angle of entry  gets very significantly less as the waterline moves aft!  (5 deg. at the transom is essentially FLAT) Therefore, the 'ride' will become more and more 'rough'/HARD/pounding in any kind of chop or waves, since the angle of entry is moved to lower deadrise and flatter hull sections.  Not sure of your area of operations or water conditions; but the only angle by the bow this hull will ride 'smoothly' is when the bow's pitch is low, and the forefoot is used to enter the seas- regardless of how tall or short they may be?

Please let me know what I could help discuss as you plan your project?

Cheers,
Kevin Morin
Kenai, AK

 
kmorin
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